MIXER (Mime Internet X.400 Enhanced Relay): Mapping between X.400 and RFC 822/MIME
RFC 2156

Network Working Group S. Kille
Request for Comments: 2156 Isode Ltd.
Obsoletes: 987, 1026, 1138, 1148, 1327, 1495 January 1998
Updates: 822
Category: Standards Track
 MIXER (Mime Internet X.400 Enhanced Relay):
 Mapping between X.400 and RFC 822/MIME
Status of this Memo
 This document specifies an Internet standards track protocol for the
 Internet community, and requests discussion and suggestions for
 improvements. Please refer to the current edition of the "Internet
 Official Protocol Standards" (STD 1) for the standardization state
 and status of this protocol. Distribution of this memo is unlimited.
Copyright Notice
 Copyright (C) The Internet Society (1998). All Rights Reserved.
Table of Contents
 1 - Overview ...................................... 3
 1.1 - X.400 ......................................... 3
 1.2 - RFC 822 and MIME .............................. 3
 1.3 - The need for conversion ....................... 4
 1.4 - General approach .............................. 4
 1.5 - Gatewaying Model .............................. 5
 1.6 - Support of X.400 (1984) ....................... 8
 1.7 - X.400 (1992) .................................. 8
 1.8 - MIME .......................................... 8
 1.9 - Body Parts .................................... 8
 1.10 - Local and Global Scenarios .................... 9
 1.11 - Compatibility with previous versions .......... 10
 1.12 - Aspects not covered ........................... 10
 1.13 - Subsetting .................................... 11
 1.14 - Specification Language ........................ 11
 1.15 - Related Specifications ........................ 11
 1.16 - Document Structure ............................ 12
 1.17 - Acknowledgements .............................. 12
 2 - Service Elements .............................. 13
 2.1 - The Notion of Service Across a Gateway ........ 13
 2.2 - RFC 822 ....................................... 15
 2.3 - X.400 ......................................... 18
 3 - Basic Mappings ................................ 27
 3.1 - Notation ...................................... 27
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 3.2 - ASCII and IA5 ................................. 29
 3.3 - Standard Types ................................ 29
 3.4 - Encoding ASCII in Printable String ............ 33
 3.5 - RFC 1522 ...................................... 34
 4 - Addressing and Message IDs .................... 35
 4.1 - A textual representation of MTS.ORAddress ..... 36
 4.2 - Global Address Mapping ........................ 43
 4.3 - EBNF.822-address <-> MTS.ORAddress ............ 46
 4.4 - Repeated Mappings ............................. 59
 4.5 - Directory Names ............................... 62
 4.6 - MTS Mappings .................................. 62
 4.7 - IPMS Mappings ................................. 67
 5 - Detailed Mappings ............................. 71
 5.1 - RFC 822 -> X.400: Detailed Mappings ........... 71
 5.2 - Return of Contents ............................ 86
 5.3 - X.400 -> RFC 822: Detailed Mappings ........... 86
 Appendix A - Mappings Specific to SMTP ..................... 114
 1 - Probes ........................................ 114
 2 - Long Lines .................................... 114
 3 - SMTP Extensions ............................... 114
 3.1 - SMTP Extension mapping to X.400 ............... 114
 3.2 - X.400 Mapping to SMTP Extensions .............. 115
 Appendix B - Mapping with X.400(1984) ...................... 116
 Appendix C - RFC 822 Extensions for X.400 access ........... 118
 Appendix D - Object Identifier Assignment .................. 119
 Appendix E - BNF Summary ................................... 120
 Appendix F - Text format for MCGAM distribution ............ 127
 1 - Text Formats .................................. 127
 2 - Mechanisms to register and to distribute
 MCGAMs ........................................ 127
 3 - Syntax Definitions ............................ 128
 4 - Table Lookups ................................. 129
 5 - Domain -> OR Address MCGAM format ............. 129
 6 - OR Address -> Domain MCGAM format ............. 129
 7 - Domain -> OR Address of Preferred Gateway
 table ......................................... 130
 8 - OR Addresss -> domain of Preferred Gateway
 table ......................................... 130
 Appendix G - Conformance ................................... 131
 Appendix H - Change History: RFC 987, 1026, 1138, 1148
 ............................................... 133
 1 - Introduction .................................. 133
 2 - Service Elements .............................. 133
 3 - Basic Mappings ................................ 133
 4 - Addressing .................................... 134
 5 - Detailed Mappings ............................. 134
 6 - Appendices .................................... 134
 Appendix I - Change History: RFC 1148 to RFC 1327 .......... 135
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 1 - General ....................................... 135
 2 - Basic Mappings ................................ 135
 3 - Addressing .................................... 135
 4 - Detailed Mappings ............................. 135
 5 - Appendices .................................... 136
 Appendix J - Change History: RFC 1327 to this Document
 ............................................... 137
 1 - General ....................................... 137
 2 - Service Elements .............................. 137
 3 - Basic Mappings ................................ 137
 4 - Addressing .................................... 137
 5 - Detailed Mappings ............................. 138
 6 - Appendices .................................... 138
 Appendix L - ASN.1 Summary ................................. 139
 Security Considerations .................................... 141
 Author's Address ........................................... 141
 References ................................................. 141
 Full Copyright Statement ................................... 144
Chapter 1 -- Overview
1.1. X.400
 This document relates primarily to the ITU-T 1988 and 1992 X.400
 Series Recommendations / ISO IEC 10021 International Standard. This
 ISO/ITU-T standard is referred to in this document as "X.400", which
 is a convenient shorthand. Any reference to the 1984 Recommendations
 will be explicit. Any mappings relating to elements which are in the
 1992 version and not in the 1988 version will be noted explicitly.
 X.400 defines an Interpersonal Messaging System (IPMS), making use of
 a store and forward Message Transfer System. This document relates
 to the IPMS, and not to wider application of X.400, such as EDI as
 defined in X.435.
1.2. RFC 822 and MIME
 RFC 822 evolved as a messaging standard on the DARPA (the US Defense
 Advanced Research Projects Agency) Internet. RFC 822 specifies an
 end to end message format, consisting of a header and an unstructured
 text body. MIME (Multipurpose Internet Mail Extensions) specifies a
 structured message body format for use with RFC 822. The term "RFC
 822" is used in this document to refer to the combination of MIME and
 RFC 822. RFC 822 and MIME are used in conjunction with a number of
 different message transfer protocol environments. The core of the
 MIXER specification is designed to work with any supporting message
 transfer protocol.
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 One transfer protocol, SMTP, is of particular importance and is
 covered in MIXER. On the Internet and other TCP/IP networks, RFC 822
 is used in conjunction with RFC 821, also known as Simple Mail
 Transfer Protocol (SMTP) [30], in a manner conformant with the host
 requirements specification [10]. Use of MIXER with SMTP is defined
 in Appendix A.
1.3. The need for conversion
 There is a large community using RFC 822 based protocols for mail
 services, who will wish to communicate with users of the IPMS
 provided by X.400 systems. This will also be a requirement in cases
 where communities intend to make a transition between the different
 technologies, as conversion will be needed to ensure a smooth service
 transition. It is expected that there will be more than one gateway,
 and this specification will enable them to behave in a consistent
 manner. Note that the term gateway is used to describe a component
 performing the mapping between RFC 822 and X.400. This is standard
 usage amongst mail implementors, but differs from that used by
 transport and network service implementors.
 Consistency between gateways is desirable to provide:
 1. Consistent service to users.
 2. The best service in cases where a message passes through
 multiple gateways.
1.4. General approach
 There are a number of basic principles underlying the details of the
 specification. These principles are goals, and are not achieved in
 all aspects of the specification.
 1. The specification should be pragmatic. There should not be
 a requirement for complex mappings for "Academic" reasons.
 Complex mappings should not be required to support trivial
 additional functionality.
 2. Subject to 1), functionality across a gateway should be as
 high as possible.
 3. It is always a bad idea to lose information as a result of
 any transformation. Hence, it is a bad idea for a gateway
 to discard information in the objects it processes. This
 includes requested services which cannot be fully mapped.
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 4. Mail gateways operate at a level above the layer on which
 they perform mappings. This implies that the gateway shall
 not only be cognisant of the semantics of objects at the
 gateway level, but also be cognisant of higher level
 semantics. If meaningful transformation of the objects that
 the gateway operates on is to occur, then the gateway needs
 to understand more than the objects themselves.
 5. Subject to 1), the mapping should be reversible. That is, a
 double transformation should bring you back to where you
 started.
1.5. Gatewaying Model
1.5.1. X.400
 X.400 defines the IPMS Abstract Service in X.420 , [11] which
 comprises of three basic services:
 1. Origination
 2. Reception
 3. Management
 Management is a local interaction between the user and the IPMS, and
 is therefore not relevant to gatewaying. The first two services
 consist of operations to originate and receive the following two
 objects:
 1. IPM (Interpersonal Message). This has two components: a
 heading, and a body. The body is structured as a sequence
 of body parts, which may be basic components (e.g., IA5
 text, or G3 fax), or forwarded Interpersonal Messages. The
 heading consists of fields containing end to end user
 information, such as subject, primary recipients (To:), and
 importance.
 2. IPN (Inter Personal Notification). A notification about
 receipt of a given IPM at the UA level.
 The Origination service also allows for origination of a probe, which
 is an object to test whether a given IPM could be correctly received.
 The Reception service also allows for receipt of Delivery Reports
 (DR), which indicate delivery success or failure.
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 These IPMS Services utilise the Message Transfer System (MTS)
 Abstract Service [12]. The MTS Abstract Service provides the
 following three basic services:
 1. Submission (used by IPMS Origination)
 2. Delivery (used by IPMS Reception)
 3. Administration (used by IPMS Management)
 Administration is a local issue, and so does not affect this
 standard. Submission and delivery relate primarily to the MTS
 Message (comprising Envelope and Content), which carries an IPM or
 IPN (or other uninterpreted contents). The Envelope includes a
 message identifier, an originator, and a list of recipients.
 Submission also includes the probe service, which supports the MTS
 Probe. Delivery also includes Reports, which indicate whether a given
 MTS Message has been delivered or not (or for a probe if delivery
 would have happened).
 The MTS is provided by MTAs which interact using the MTA (Message
 Transfer Agent) Service, which defines the interaction between MTAs,
 along with the procedures for distributed operation. This service
 provides for transfer of MTS Messages, Probes, and Reports.
1.5.2. RFC 822
 RFC 822 is based on the assumption that there is an underlying
 service, which is here called the 822-MTS service. The 822-MTS
 service provides three basic functions:
 1. Identification of a list of recipients.
 2. Identification of an error return address.
 3. Transfer of an RFC 822 message.
 It is possible to achieve 2) within the RFC 822 header.
 This specification will be used most commonly with SMTP as the 822-
 MTS service. The core MIXER specification is written so that it does
 not rely on non-basic 822-MTS services. Use of non-basic SMTP
 services is described in Appendix A. The core of this document is
 written using SMTP terminology for 822-MTS services.
 An RFC 822 message consists of a header, and content which is
 uninterpreted ASCII text. The header is divided into fields, which
 are the protocol elements. Most of these fields are analogous to IPM
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 heading fields, although some are analogous to MTS Service Elements
 or MTA Service Elements.
 RFC 822 supports delivery status notifications by use of the NOTARY
 mechanisms [28].
1.5.3. The Gateway
 Given this functional description of the two services, the functional
 nature of a gateway can now be considered. It would be elegant to
 consider the SMTP (822-MTS) service mapping onto the MTS Service
 Elements and RFC 822 mapping onto an IPM, but there is a not a clear
 match between these services. Another elegant approach would be to
 treat this document as the definition of an X.400 Access Unit (AU).
 In this case, the abstraction level is too high, and some necessary
 mapping function is lost. It is necessary to consider that the IPM
 format definition, the IPMS Service Elements, the MTS Service
 Elements, and MTA Service Elements on one side are mapped into RFC
 822 + SMTP on the other in a slightly tangled manner. The details of
 the tangle will be made clear in Chapter 5. Access to the MTA
 Service Elements is minimised.
 The following basic mappings are thus defined. When going from RFC
 822 to X.400, an RFC 822 message and the associated SMTP information
 is always mapped into an IPM (MTA, MTS, and IPMS Services) and a
 Delivery Status Notification is mapped onto a Report. Going from
 X.400 to RFC 822, an RFC 822 message and the associated SMTP
 information may be derived from:
 1. An IPN (MTA, MTS, and IPMS services)
 2. An IPM (MTA, MTS, and IPMS services)
 A Report (MTA, and MTS Services) is mapped onto a delivery status
 notification.
 Probes (MTA Service) shall be processed by the gateway, as discussed
 in Chapter 5. MTS Messages containing Content Types other than those
 defined by the IPMS are not mapped by the gateway, and shall be
 rejected at the gateway if no other gatewaying procedure is defined.
 This specification is concerned with X.400 IPMS. Future
 specifications may defined mappings for other X.400 content types.
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1.5.4. Repeated Mappings
 The primary goal of this specification is to support single mappings,
 so that X.400 and RFC 822 users can communicate with maximum
 functionality.
 The mappings specified here are designed to work where a message
 traverses multiple times between X.400 and RFC 822. This is often
 essential, particularly in the case of distribution lists. However,
 in general, this will lead to a level of service which is the lowest
 common denominator (approximately the services offered by RFC 822).
 Some RFC 822 networks may wish to use X.400 as an interconnection
 mechanism (typically for policy reasons), and this is fully
 supported.
 Where an X.400 message transfers to RFC 822 and then back to X.400,
 there is no expectation of X.400 services which do not have an
 equivalent service in standard RFC 822 being preserved - although
 this may be possible in some cases.
1.6. Support of X.400 (1984)
 The MIXER definition is based on the initial specification of RFC 987
 and in its addendum RFC 1026, which defined a mapping between
 X.400(1984) and RFC 822. The core MIXER mapping is defined using the
 full 1988 version of X.400, and not to a 1984 compatible subset. New
 features of X.400(1988) can be used to provide a much cleaner mapping
 than that defined in RFC 987. To interwork with 1984 systems,
 Appendix B shall be followed.
 If a message is being transferred to an X.400(1984) system by way of
 X.400(1988) MTA it will give a slightly better service to follow the
 rules of Appendix B, than to downgrade without this knowledge.
 Downgrading specifications which supplement those specified in X.400
 (X.419) are given in RFC 1328 [22] and RFC 1496 (HARPOON) [5].
1.7. X.400 (1992)
 X.400 (1992) features are not used by the core of this mapping, and
 so there is not an equivalent downgrade problem.
1.8. MIME
 MIME format messages are generated by this mapping. As MIME messages
 are fully RFC 822 compliant, this will not cause problems with
 systems which are not MIME capable.
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1.9. Body Parts
 MIME and X.400 IPMS can both carry arbitrary body parts. MIME defines
 a mechanism for adding new body parts, and new body parts are
 registered with the IANA. X.400 defines a mechanism adding new body
 parts, usually referred to as Body Part 15. Extensions are defined
 by Object Identifiers, so there is no requirement for a central body
 part registration authority. The Electronic Messaging Association
 (EMA) maintains a list of some commonly used body parts. The EMA has
 specified a mechanism to use the File Transfer Body Part (FTBP) as a
 more generic means to support message attachments. This approach is
 gaining widespread commercial support.
 The mapping between X.400 and MIME body parts is defined in the
 companion MIXER specification, referenced here as RFC 2157 [8]. This
 document is an update of RFC 1494 [6].
 Editor's Note:
 References to 2157 will be resolved as these two
 documents are expected to progress in parallel.
 These two specifications together form the complete MIXER Mapping.
1.10. Local and Global Scenarios
 There are two basic scenarios for X.400/MIME interworking:
 Global Scenario
 There are two global mail networks (Internet/MIME and X.400),
 interconnected by multiple gateways. Objects may be transferred
 over multiple gateways, and so it is important that gateways
 behave in a coherent fashion. MIXER is critical to support this
 scenario.
 Local Scenario
 A gateway is used to connect a closed community to a global mail
 network (this could be enforced by connectivity or gateway
 authorisation policy). This is a common commercial scenario.
 MIXER is useful to support this scenario, as it allows an industry
 standard provision of service, but this could be supported by
 something which was MIXER-like.
 A solution for the global scenario will work for the local scenario.
 However, there are aspects of MIXER which have significant
 implementation or deployment effort (the global mapping is the major
 one, but there are other details too) which and are needed to support
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 the global scenario, but are not needed in the local scenario.
 Note that the local scenario may be the driving force for most
 deployments, and support of the global scenario may be an important
 secondary goal.
 There is also a transition effect. Gateways which are initially
 deployed in a strict local scenario situation start to find
 themselves in a global scenario. A common case is ADMD provided
 gateways, which are targeted strictly at the local scenario. In
 practice they soon start to operate in the global scenario, because
 of distribution lists and messages exchanged with X.400 users that
 are not customers of the ADMD. At this point, users are hurt by the
 restrictions of a local scenario gateway.
 Note that conformance to MIXER applies to an instantiation of a
 gateway, not just an implementation (although clearly it is critical
 that the implementation is capable of being operated in a conformant
 manner).
 MIXER's conformance target is the global scenario, and the
 specification of MIXER defines operation in this way.
1.11. Compatibility with previous versions
 The changes between this and older versions of the document are given
 in Appendices H, I and J. These are RFCs 987, 1026, 1138, 1148 and
 1327. This document is a revision of RFC 1327 [21]. As far as
 possible, changes have been made in a compatible fashion.
1.12. Aspects not covered
 There have been a number of cases where previous versions of this
 document were used in a manner which was not intended. This section
 is to make clear some limitations of scope. In particular, this
 specification does not specify:
 - Extensions of RFC 822 to provide access to all X.400
 services
 - X.400 user interface definition
 These are really coupled. To map the X.400 services, this
 specification defines a number of extensions to RFC 822. As a side
 effect, these give the 822 user access to SOME X.400 services.
 However, the aim on the RFC 822 side is to preserve current service,
 and it is intentional that access is not given to all X.400 services.
 Thus, it will be a poor choice for X.400 implementors to use MIXER as
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 an interface - there are too many aspects of X.400 which cannot be
 accessed through it. If a text interface is desired, a specification
 targeted at X.400, without RFC 822 restrictions, would be more
 appropriate. Some optional and limited extensions in this area have
 proved useful, and are defined in Appendix C.
1.13. Subsetting
 This proposal specifies a mapping which is appropriate to preserve
 services in existing RFC 822 communities. Implementations and
 specifications which subset this specification are non-conformant and
 strongly discouraged.
1.14. Specification Language
 ISO and Internet standards have clear definitions as to the style of
 language used. This specification maps between ISO/ITU-T protocol
 and Internet protocols. This document uses ISO terminology for the
 following reasons:
 1. This was done in previous versions.
 2. ISO language may be mechanically converted to Internet
 language, but not vice versa.
 The key elements of the ISO rules are:
 1. All mandatory features shall clearly be indicated by
 imperative statements or the word "shall" or "shall not".
 2. Optional features shall be indicated by the word "may".
 3. The word "should" and the phrase "may not" shall not be
 used.
 In some cases the specification issues guidance on use of optional
 features, by use of the the phrase word "recommended" or "not
 recommended".
 To interpet this document according to Internet rules, replace every
 occurrence of "shall" with "must".
1.15. Related Specifications
 Mappings between Mail-11 and X.400 and Mail-11 and RFC 822 are
 described in RFC 2162, using mappings related to those defined here
 [2].
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1.16. Document Structure
 This document has five chapters:
 1. Overview - this chapter.
 2. Service Elements - This describes the (end user) services
 mapped by a gateway.
 3. Basic mappings - This describes some basic notation used in
 Chapters 3-5, the mappings between character sets, and some
 fundamental protocol elements.
 4. Addressing - This considers the mapping between X.400 OR
 names and RFC 822 addresses, which is a fundamental gateway
 component.
 5. Detailed Mappings - This describes the details of all other
 mappings.
 There are also ten appendices.
 WARNING:
 THE REMAINDER OF THIS SPECIFICATION IS TECHNICALLY DETAILED. IT
 WILL NOT MAKE SENSE, EXCEPT IN THE CONTEXT OF RFC 822 AND X.400
 (1988). DO NOT ATTEMPT TO READ THIS DOCUMENT UNLESS YOU ARE
 FAMILIAR WITH THESE SPECIFICATIONS.
1.17. Acknowledgements
 The work in this specification was substantially based on RFC 987 and
 RFC 1148, which had input from many people, who are credited in the
 respective documents.
 A number of comments from people on RFC 1148 lead to RFC 1327. In
 particular, there were comments and suggestions from: Maurice Abraham
 (HP); Harald Alvestrand (Sintef); Peter Cowen (X-Tel); Jim Craigie
 (JNT); Ella Gardner (MITRE); Christian Huitema (Inria); Erik Huizer
 (SURFnet); Neil Jones (DEC); Ignacio Martinez (IRIS); Julian Onions
 (X-Tel); Simon Poole (SWITCH); Clive Roberts (Data General); Pete
 Vanderbilt (SUN); Alan Young (Concurrent).
 RFC 1327 has been widely adopted, and a review team was formed. This
 comprised of: Urs Eppenberger (SWITCH)(Chair); Claudio Allocchio
 (INFN); Harald Alvestrand (UNINETT); Dave Crocker (Brandenburg); Ned
 Freed (Innosoft); Erik Huizer (SURFnet); Steve Kille (Isode); Peter
 Sylvester (GC Tech).
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 Harald Alvestrand also supplied the tables mapping DSN status codes
 with X.400 codes. Ned Freed defined parts of the File Transfer Body
 Part mapping.
 Comment and input has also been received from: Bengt Ackzell (Generic
 Systems); Samir Albadine (Transpac); Mark Boyes (DEC); Larry Campbell
 (Boston Software Works); Jacqui Caren (Cray); Allan Cargille (MCI);
 Kevin Carrosso (Innosoft); Charlie Combs (OIW); Jim Craigie (Net-
 Tel); Eamon Doyle (Isocor); Efifion Edem (SITA); Jyrki Heikkinen
 (ICL); Edward Hibbert (DCL); Jeroun Houttin (Terena); Kevin Jordan
 (CDS); Paul Kingsnorth (DEC); Carl-Uno Manros (Manros Consulting);
 Suzan Mendes (Telis); Robert Miles (Softswitch); Roger Mizumorri
 (Enterprise Solutions Ltd); Keith Moore (University of Tennessee);
 Ruth Moulton (Net-Tel) Michel Musy (Bull); Kenji Nonaka (NTT): The
 OIW MHSIG; Tom Oliphant (SWITCH); Julian Onions (NEXOR); Jacob Palme
 (KTH); Olivier Paridaens (ULB); Mary la Roche (Citicorp); John
 Setsaas (Maxware); Russell Sharpe (DCL); Patrick Soulier (CCETT);
 Eftimios Tsigros (Universite Libre de Bruxelles); Sean Turner (IECA);
 Mark Wahl (Isode); David Wilson (Isode); Bill Wohler (Worldtalk);
 Alan Young (Isode); Alain Zahm (Telis).
Chapter 2 - Service Elements
 This chapter considers the services offered across a gateway built
 according to this specification. It gives a view of the
 functionality provided by such a gateway for communication with users
 in the opposite domain. This chapter considers service mappings in
 the context of SINGLE transfers only, and not repeated mappings
 through multiple gateways.
2.1. The Notion of Service Across a Gateway
 RFC 822 and X.400 provide a number of services to the end user. This
 chapter describes the extent to which each service can be supported
 across an X.400 <-> RFC 822 gateway. The cases considered are single
 transfers across such a gateway, although the problems of multiple
 crossings are noted where appropriate.
2.1.1. Origination of Messages
 When a user originates a message, a number of services are available.
 Some of these imply actions (e.g., delivery to a recipient), and some
 are insertion of known data (e.g., specification of a subject field).
 This chapter describes, for each offered service, to what extent it
 is supported for a recipient accessed through a gateway. There are
 three levels of support:
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 Supported
 The corresponding protocol elements map well, and so the service
 can be fully provided.
 Not Supported
 The service cannot be provided, as there is a complete mismatch.
 Partial Support
 The service can be partially fulfilled.
 In the first two cases, the service is simply marked as "Supported"
 or "Not Supported". Some explanation may be given if there are
 additional implications, or the (non) support is not intuitive. For
 partial support, the level of partial support is summarised. Where
 partial support is good, this will be described by a phrase such as
 "Supported by use of.....". A common case of this is where the
 service is mapped onto a non-standard service on the other side of
 the gateway, and this would have lead to support if it had been a
 standard service. In many cases, this is equivalent to support. For
 partial support, an indication of the mechanism is given, in order to
 give a feel for the level of support provided. Note that this is not
 a replacement for Chapter 5, where the mapping is fully specified.
 If a service is described as supported, this implies:
 - Semantic correspondence.
 - No (significant) loss of information.
 - Any actions required by the service element.
 An example of a service gaining full support: If an RFC 822
 originator specifies a Subject: field, this is considered to be
 supported, as an X.400 recipient will get a subject indication.
 In many cases, the required action will simply be to make the
 information available to the end user. In other cases, actions may
 imply generating a delivery report.
 All RFC 822 services are supported or partially supported for
 origination. The implications of non-supported X.400 services is
 described under X.400.
2.1.2. Reception of Messages
 For reception, the list of service elements required to support this
 mapping is specified. This is really an indication of what a
 recipient might expect to see in a message which has been remotely
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 originated.
2.2. RFC 822
 RFC 822 does not explicitly define service elements, as distinct from
 protocol elements. However, all of the RFC 822 header fields, with
 the exception of trace, can be regarded as corresponding to implicit
 RFC 822 service elements.
2.2.1. Origination in RFC 822
 A mechanism of mapping, used in several cases, is to map the RFC 822
 header into a heading extension in the IPM (InterPersonal Message).
 This can be regarded as partial support, as it makes the information
 available to any X.400 implementations which are interested in these
 services. Communities which require significant RFC 822 interworking
 are recommended to require that their X.400 User Agents are able to
 display these heading extensions. Support for the various service
 elements (headers) is now listed.
 Date:
 Supported.
 From:
 Supported. For messages where there is also a sender field,
 the mapping is to "Authorising Users Indication", which has
 subtly different semantics to the general RFC 822 usage of
 From:.
 Sender: Supported.
 Reply-To: Supported.
 To: Supported.
 Cc: Supported.
 Bcc: Supported.
 Message-Id: Supported.
 In-Reply-To:
 Supported, for a single reference. Where multiple references are
 given, partial support is given by mapping to "Cross Referencing
 Indication". This gives similar semantics.
 References: Supported.
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 Keywords: Supported by use of a heading extension.
 Subject: Supported.
 Comments: Supported by use of a heading extension.
 Encrypted: Supported by use of a heading extension.
 Content-Language: Supported.
 Resent-*
 Supported by use of a heading extension. Note that addresses in
 these fields are mapped onto text, and so are not accessible to
 the X.400 user as addresses. In principle, fuller support would
 be possible by mapping onto a forwarded IP Message, but this is
 not suggested.
 Other Fields
 In particular X-* fields, and "illegal" fields in common usage
 (e.g., "Fruit-of-the-day:") are supported by use of heading
 extensions.
 MIME introduces a number of headings. Support is defined in RFC
 2157.
2.2.2. Reception by RFC 822
 This considers reception by an RFC 822 User Agent of a message
 originated in an X.400 system and transferred across a gateway. The
 following standard services (headers) may be present in such a
 message:
 Date:
 From:
 Sender:
 Reply-To:
 To:
 Cc:
 Bcc:
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 Message-Id:
 In-Reply-To:
 References:
 Subject:
 Content-Type: (See RFC 2157)
 Content-Transfer-Encoding: (See RFC 2157)
 MIME-Version: (See RFC 2157)
 The following services (headers) may be present in the header of a
 message. These are defined in more detail in Chapter 5 (5.3.4, 5.3.6,
 5.3.7):
 Autoforwarded:
 Autosubmitted:
 X400-Content-Identifier:
 Content-Language:
 Conversion:
 Conversion-With-Loss:
 Delivery-Date:
 Discarded-X400-IPMS-Extensions:
 Discarded-X400-MTS-Extensions:
 DL-Expansion-History:
 Deferred-Delivery:
 Expires:
 Importance:
 Incomplete-Copy:
 Latest-Delivery-Time:
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 Message-Type:
 Original-Encoded-Information-Types:
 Originator-Return-Address:
 Priority:
 Reply-By:
 Sensitivity:
 Supersedes:
 X400-Content-Type:
 X400-MTS-Identifier:
 X400-Originator:
 X400-Received:
 X400-Recipients:
2.3. X.400
2.3.1. Origination in X.400
 When mapping services from X.400 to RFC 822 which are not supported
 by RFC 822, new RFC 822 headers are defined, and registered by
 publication in this standard. It is intended that co-operating RFC
 822 systems may also use them. Where these new fields are used, and
 no system action is implied, the service can be regarded as being
 partially supported. Chapter 5 describes how to map X.400 services
 onto these new headers. Other elements are provided, in part, by the
 gateway as they cannot be provided by RFC 822.
 Some service elements are marked N/A (not applicable). There are
 five cases, which are marked with different comments:
 N/A (local)
 These elements are only applicable to User Agent / Message
 Transfer Agent interaction and so they cannot apply to RFC 822
 recipients.
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 N/A (PDAU)
 These service elements are only applicable where the recipient is
 reached by use of a Physical Delivery Access Unit (PDAU), and so
 do not need to be mapped by the gateway.
 N/A (reception)
 These services are only applicable for reception.
 N/A (prior)
 If requested, this service shall be performed prior to the
 gateway.
 N/A (MS)
 These services are only applicable to Message Store (i.e., a local
 service).
 Finally, some service elements are not supported. In particular, the
 new security services are not mapped onto RFC 822. Unless otherwise
 indicated, the behaviour of service elements marked as not supported
 will depend on the criticality marking supplied by the user. If the
 element is marked as critical for transfer or delivery, a non-
 delivery notification will be generated. Otherwise, the service
 request will be ignored.
2.3.1.1. Basic Interpersonal Messaging Service
 These are the mandatory IPM services as listed in Section 19.8 of
 X.400 / ISO/IEC 10021-1, listed here in the order given. Section 19.8
 has cross references to short definitions of each service.
 Access management
 N/A (local).
 Content Type Indication
 Supported by a new RFC 822 header (X400-Content-Type:).
 Converted Indication
 Supported by a new RFC 822 header (X400-Received:).
 Delivery Time Stamp Indication
 N/A (reception).
 IP Message Identification
 Supported.
 Message Identification
 Supported, by use of a new RFC 822 header (X400-MTS-Identifier).
 This new header is required, as X.400 has two message-ids whereas
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 RFC 822 has only one (see IP Message Identification
 Non-delivery Notification
 Not supported in all cases. Supported where the recipient system
 supports NOTARY DSNs. In general all RFC 822 systems will return
 error reports by use of IP messages. In other service elements,
 this pragmatic result can be treated as effective support of this
 service element.
 Original Encoded Information Types Indication
 Supported as a new RFC 822 header (Original-Encoded-Information-
 Types:).
 Submission Time Stamp Indication
 Supported.
 Typed Body
 Support is defined in RFC 2157.
 User Capabilities Registration
 N/A (local).
2.3.1.2. IPM Service Optional User Facilities
 This section describes support for the optional (user selectable) IPM
 services as listed in Section 19.9 of X.400 / ISO/IEC 10021- 1,
 listed here in the order given. Section 19.9 has cross references to
 short definitions of each service.
 Additional Physical Rendition
 N/A (PDAU).
 Alternate Recipient Allowed
 Not supported. There is no RFC 822 service equivalent to
 prohibition of alternate recipient assignment (e.g., an RFC 822
 system may freely send an undeliverable message to a local
 postmaster). A MIXER gateway has two conformant options. The
 first is not to gateway a message requesting prohibition of
 alternate recipient, as this control cannot be guaranteed. This
 option supports the service, but may cause unacceptable level of
 message rejections. The second is to gateway the message on the
 basis that there is no alternate recipient service in RFC 822. RFC
 1327 allowed only the second option. If the first option is
 shown to be operationally effective, it may be the only option in
 future versions of MIXER.
 Authorising User's Indication
 Supported.
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 Auto-forwarded Indication
 Supported as new RFC 822 header (Auto-Forwarded:).
 Basic Physical Rendition
 N/A (PDAU).
 Blind Copy Recipient Indication
 Supported.
 Body Part Encryption Indication
 Supported by use of a new RFC 822 header (Original-Encoded-
 Information-Types:), although in most cases it will not be
 possible to map the body part in question.
 Content Confidentiality
 Not supported.
 Content Integrity
 Not supported.
 Conversion Prohibition
 Supported. Operation defined in RFC 2157.
 Conversion Prohibition in Case of Loss of Information
 Supported. Operation defined in RFC 2157.
 Counter Collection
 N/A (PDAU).
 Counter Collection with Advice
 N/A (PDAU).
 Cross Referencing Indication
 Supported.
 Deferred Delivery
 N/A (prior). This service shall always be provided by the MTS
 prior to the gateway. A new RFC 822 header (Deferred-Delivery:)
 is provided to transfer information on this service to the
 recipient.
 Deferred Delivery Cancellation
 N/A (local).
 Delivery Notification
 Supported. This is performed at the gateway, but may be performed
 at the end system if the end system supports NOTARY. Thus, a
 notification is sent by the gateway to the originator.
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 Delivery via Bureaufax Service
 N/A (PDAU).
 Designation of Recipient by Directory Name
 N/A (local).
 Disclosure of Other Recipients
 Supported by use of a new RFC 822 header (X400-Recipients:). This
 is descriptive information for the RFC 822 recipient, and is not
 reverse mappable.
 DL Expansion History Indication
 Supported by use of a new RFC 822 header (DL-Expansion-History:).
 DL Expansion Prohibited
 Distribution List means MTS supported distribution list, in the
 manner of X.400. This service does not exist in the RFC 822
 world, although RFC 822 supports distribution list functionality.
 There is no SMTP leve control to prohibit distribution list
 expansion. A MIXER gateway has two conformant options. The
 first is not to gateway a message requesting DL expansion
 prohibition, as this control cannot be guaranteed. This option
 supports the service, but may cause unacceptable level of message
 rejections. The second is to gateway the message on the basis that
 there is no distribution list service in RFC 822. RFC 1327 allowed
 only the second option. If the first option is shown to be
 operationally effective, it may be the only option in future
 versions of MIXER.
 Express Mail Service
 N/A (PDAU).
 Expiry Date Indication
 Supported as new RFC 822 header (Expires:). In general, no
 automatic action can be expected.
 Explicit Conversion
 N/A (prior).
 Forwarded IP Message Indication
 Supported.
 Grade of Delivery Selection
 Not Supported. There is no equivalent service in RFC 822.
 Importance Indication
 Supported as new RFC 822 header (Importance:).
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 Incomplete Copy Indication
 Supported as new RFC 822 header (Incomplete-Copy:).
 Language Indication
 Supported as new RFC 822 header (Content-Language:).
 Latest Delivery Designation
 Not supported. A new RFC 822 header (Latest-Delivery-Time:) is
 provided, which may be used by the recipient for general
 information, but will not be acted on by the SMTP infrastrucuture.
 Message Flow Confidentiality
 Not supported.
 Message Origin Authentication
 N/A (reception).
 Message Security Labelling
 Not supported.
 Message Sequence Integrity
 Not supported.
 Multi-Destination Delivery Supported.
 Multi-part Body
 Supported.
 Non Receipt Notification Request
 Not supported.
 Non Repudiation of Delivery
 Not supported.
 Non Repudiation of Origin
 N/A (reception).
 Non Repudiation of Submission
 N/A (local).
 Obsoleting Indication
 Supported as new RFC 822 header (Supersedes:).
 Ordinary Mail
 N/A (PDAU).
 Originator Indication
 Supported.
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 Originator Requested Alternate Recipient
 Not supported, but is placed as comment next to address (X400-
 Recipients:).
 Physical Delivery Notification by MHS
 N/A (PDAU).
 Physical Delivery Notification by PDS
 N/A (PDAU).
 Physical Forwarding Allowed
 Supported by use of a comment in a new RFC 822 header (X400-
 Recipients:), associated with the recipient in question.
 Physical Forwarding Prohibited
 Supported by use of a comment in a new RFC 822 header (X400-
 Recipients:), associated with the recipient in question.
 Prevention of Non-delivery notification
 Supported where SMTP and NOTARY are available. In other cases
 formally supported, as delivery notifications cannot be generated
 by RFC 822. In practice, errors will be returned as IP Messages,
 and so this service may appear not to be supported (see Non-
 delivery Notification).
 Primary and Copy Recipients Indication
 Supported
 Probe
 Supported at the gateway (i.e., the gateway services the probe).
 Probe Origin Authentication
 N/A (reception).
 Proof of Delivery
 Not supported.
 Proof of Submission
 N/A (local).
 Receipt Notification Request Indication
 Not supported.
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 Redirection Disallowed by Originator
 Redirection means MTS supported redirection, in the manner of
 X.400. This service does not exist in the RFC 822 world. RFC 822
 redirection (e.g., aliasing) is regarded as an informal
 redirection mechanism, beyond the scope of this control. Messages
 will be sent to RFC 822, irrespective of whether this service is
 requested. In practice, control of this service is not supported.
 Registered Mail
 N/A (PDAU).
 Registered Mail to Addressee in Person
 N/A (PDAU).
 Reply Request Indication
 Supported as comment next to address.
 Replying IP Message Indication
 Supported.
 Report Origin Authentication
 N/A (reception).
 Request for Forwarding Address
 N/A (PDAU).
 Requested Delivery Method
 N/A (local). The service request is dealt with at submission
 time. Any such request is made available through the gateway by
 use of a comment associated with the recipient in question.
 Return of Content
 Supported where SMTP and NOTARY are used. In principle for other
 situations, this is N/A, as non-delivery notifications are not
 supported. In practice, most RFC 822 systems will return part or
 all of the content along with the IP Message indicating an error
 (see Non-delivery Notification).
 Sensitivity Indication
 Supported as new RFC 822 header (Sensitivity:).
 Special Delivery
 N/A (PDAU).
 Stored Message Deletion
 N/A (MS).
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 Stored Message Fetching
 N/A (MS).
 Stored Message Listing
 N/A (MS).
 Stored Message Summary
 N/A (MS).
 Subject Indication
 Supported.
 Undeliverable Mail with Return of Physical Message
 N/A (PDAU).
 Use of Distribution List
 In principle this applies only to X.400 supported distribution
 lists (see DL Expansion Prohibited). Theoretically, this service
 is N/A (prior). In practice, because of informal RFC 822 lists,
 this service can be regarded as supported.
 Auto-Submitted Indication
 Supported
2.3.2. Reception by X.400
2.3.2.1. Standard Mandatory Services
 The following standard IPM mandatory user facilities are required for
 reception of RFC 822 originated mail by an X.400 UA.
 Content Type Indication
 Delivery Time Stamp Indication
 IP Message Identification
 Message Identification
 Non-delivery Notification
 Original Encoded Information Types Indication
 Submission Time Stamp Indication
 Typed Body
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2.3.2.2. Standard Optional Services
 The following standard IPM optional user facilities are required for
 reception of RFC 822 originated mail by an X.400 UA.
 Authorising User's Indication
 Blind Copy Recipient Indication
 Cross Referencing Indication
 Originator Indication
 Primary and Copy Recipients Indication
 Replying IP Message Indication
 Subject Indication
2.3.2.3. New Services
 A new X.400 service "RFC 822 Header Field" is defined using the
 extension facilities. This allows for any RFC 822 header field to be
 represented. It may be present in RFC 822 originated messages which
 are received by an X.400 UA.
Chapter 3 Basic Mappings
3.1. Notation
 The X.400 protocols are encoded in a structured manner according to
 ASN.1, whereas RFC 822 is text encoded. To define a detailed
 mapping, it is necessary to refer to detailed protocol elements in
 each format. A notation to achieve this is described in this
 section.
3.1.1. RFC 822
 Structured text is defined according to the Extended Backus Naur Form
 (EBNF) defined in Section 2 of RFC 822 [16]. In the EBNF definitions
 used in this specification, the syntax rules given in Appendix D of
 RFC 822 are assumed. When these EBNF tokens are referred to outside
 an EBNF definition, they are identified by the string "822." appended
 to the beginning of the string (e.g., 822.addr-spec). Additional
 syntax rules, to be used throughout this specification, are defined
 in this chapter.
 The EBNF is used in two ways.
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 1. To describe components of RFC 822 messages (or of SMTP
 components). When these new EBNF tokens are referred to
 outside an EBNF definition, they are identified by the
 string "EBNF." appended to the beginning of the string
 (e.g., EBNF.importance).
 2. To describe the structure of IA5 or ASCII information not in
 an RFC 822 message.
 For all new EBNF, tokens will either be self delimiting, or be
 delimited by self delimiting tokens. Comments and LWSP are not used
 as delimiters, except for the following cases, where LWSP may be
 inserted according to RFC 822 rules.
 - Around the ":" in all headers
 - EBNF.labelled-integer
 - EBNF.object-identifier
 - EBNF.encoded-info
 RFC 822 folding rules are applied to all headers. Comments are never
 used in these new headers.
 This notation is used in a modified form to refer to NOTARY EBNF
 [28]. For this EBNF, the keyword EBNF it replaces with DSN, for
 example DSN.final-recipient-field fields.
3.1.2. ASN.1
 An element is referred to with the following syntax, defined in EBNF:
 element = service "." definition *( "." definition )
 service = "IPMS" / "MTS" / "MTA"
 definition = identifier / context
 identifier = ALPHA *< ALPHA or DIGIT or "-" >
 context = "[" 1*DIGIT "]"
 The EBNF.service keys are shorthand for the following service
 specifications:
 IPMS IPMSInformationObjects defined in Annex E of X.420 / ISO 10021-
 7.
 MTS MTSAbstractService defined in Section 9 of X.411 / ISO 10021-4.
 TA MTAAbstractService defined in Section 13 of X.411 / ISO 10021-4.
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 FTBP File Transfer Body Part, as defined in [27].
 The first EBNF.identifier identifies a type or value key in the
 context of the defined service specification. Subsequent
 EBNF.identifiers identify a value label or type in the context of the
 first identifier (SET or SEQUENCE). EBNF.context indicates a context
 tag, and is used where there is no label or type to uniquely identify
 a component. The special EBNF.identifier keyword "value" is used to
 denote an element of a sequence. For example, IPMS.Heading.subject
 defines the subject element of the IPMS heading. The same syntax is
 also used to refer to element values. For example,
 MTS.EncodedInformationTypes.[0].g3Fax refers to a value of
 MTS.EncodedInformationTypes.[0] .
3.2. ASCII and IA5
 A gateway will interpret all IA5 as ASCII. Thus, mapping between
 these forms is conceptual.
3.3. Standard Types
 There is a need to convert between ASCII text and some of the types
 defined in ASN.1 [14]. For each case, an EBNF syntax definition is
 given, for use in all of this specification, which leads to a mapping
 between ASN.1, and an EBNF construct. All EBNF syntax definitions of
 ASN.1 types are in lower case, whereas ASN.1 types are referred to
 with the first letter in upper case. Except as noted, all mappings
 are symmetrical.
3.3.1. Boolean
 Boolean is encoded as:
 boolean = "TRUE" / "FALSE"
3.3.2. NumericString
 NumericString is encoded as:
 numericstring = *(DIGIT / " ")
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3.3.3. PrintableString
 PrintableString is a restricted IA5String defined as:
 printablestring = *( ps-char )
 ps-restricted-char = 1DIGIT / 1ALPHA / " " / "'" / "+"
 / "," / "-" / "." / "/" / ":" / "=" / "?"
 ps-delim = "(" / ")"
 ps-char = ps-delim / ps-restricted-char
 This can be used to represent real printable strings in EBNF.
3.3.4. T.61String
 In cases where T.61 strings are only used for conveying human
 interpreted information, the aim of a mapping is to render the
 characters appropriately in the remote character set, rather than to
 maximise reversibility. For these cases, there are two options, both
 of which are conformant to this specification:
 1. The mappings to IA5 defined in ITU-T Recommendation X.408
 (1988) may be used [13]. These will then be encoded in
 ASCII. This is the approach mandated in RFC 1327.
 2. This mapping may be used if the characters are not contained
 within ASCII repertoire, but are all in an IANA-registered
 character set. Use the encoding defined in RFC 1522 [9] to
 generate appropriate encoded-words. If this mapping is
 used, the character set ISO-8859-1 shall be used if all of
 the characters needed are available in this repertoire. In
 other cases, the character set TELETEX shall be used. The
 details of this character set is defined in the Appendix C
 of RFC 2157.
 There is also a need to represent Teletex Strings in ASCII, for some
 aspects of OR Address. For these, the following encoding is used:
 teletex-string = *( ps-char / t61-encoded )
 t61-encoded = "{" 1* t61-encoded-char "}"
 t61-encoded-char = 3DIGIT
 Characters in EBNF.ps-char are mapped simply. Other octets,
 including control characters, are mapped using a quoting mechanism
 similar to the printable string mechanism. Each octet is represented
 as 3 decimal digits. For example, the Yen character (hex A5) is
 represented as {165}. As the three character string, a, yen
 character, b, would be represented as either "a{165}b".
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 The use of escape sequences follows that set down for ASN1. in ISO
 8825-1, with the additional specifiction that the default G1 page is
 ISO Latin 1. The page settings may be changed by escape sequences.
 Changes of the settings hold within a pair of curly brackets ({}),
 and the settings revert to the default after the right bracket (})
 (i.e., they do not carry forward to subsequent T.61 encoding).
 There are a number of places where a string may have a Teletex and/or
 Printable String representation. The following EBNF is used to
 represent this.
 teletex-and-or-ps = [ printablestring ] [ "*" teletex-string ]
 The natural mapping is restricted to EBNF.ps-char, in order to make
 the full BNF easier to parse. An example is:
 "yen*{165}"
3.3.5. UTCTime
 Both UTCTime and the RFC 822 822.date-time syntax contain: Year,
 Month, Day of Month, hour, minute, second (optional), and Timezone
 (technically a time differential in UTCTime). 822.date-time also
 contains an optional day of the week, but this is redundant. With
 the exception of Year, a symmetrical mapping can be made between
 these constructs.
 Note:
 In practice, a gateway will need to parse various illegal variants
 on 822.date-time. In cases where 822.date-time cannot be parsed,
 it is recommended that the derived UTCTime is set to the value at
 the time of translation. Such errors may be noted in an RFC 822
 comment, to aid detection and correction.
 When mapping to X.400, the UTCTime format which specifies the
 timezone offset shall be used.
 When mapping to RFC 822, the 822.date-time format shall include a
 numeric timezone offset (e.g., -0500).
 When mapping time values, the timezone shall be preserved as
 specified. The date shall not be normalised to any other timezone.
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 RFC 822, as modified by RFC 1123, requires use of a four digit year.
 Note that the original RFC 822 uses a two digit date, which is no
 longer legal. UTCTime uses a two digit date. To map a year from RFC
 822 to X.400, simply use the last two digits. To map a year from
 X.400 to RFC 822, assume that the two digit year refers to a year in
 the 10 year epoch 1980-2079.
3.3.6. Integer
 A basic ASN.1 Integer will be mapped onto EBNF.numericstring. In
 many cases ASN.1 will enumerate Integer values or use ENUMERATED. An
 EBNF encoding labelled-integer is provided. When mapping from EBNF to
 ASN.1, only the integer value is mapped, and the associated text is
 discarded. When mapping from ASN.1 to EBNF, a text label may be
 added. It is recommended that this is done wherever possible and
 that clear text labels are chosen.
 A second encoding labelled-integer-2 is provided. This is used in
 DSNs, where the parsing rules will treat the text as a comment. This
 definition was not present in RFC 1327.
 labelled-integer ::= [ key-string ] "(" numericstring ")"
 labelled-integer-2 ::= [ numericstring ] "(" key-string ")"
 key-string = *key-char
 key-char = <a-z, A-Z, 0-9, and "-">
3.3.7. Object Identifier
 Object identifiers are represented in a form similar to that given in
 ASN.1. The order is the same as for ASN.1 (big-endian). The numbers
 are mandatory, and used when mapping from the ASCII to ASN.1. The
 key-strings are optional. It is recommended that as many strings as
 possible are generated when mapping from ASN.1 to ASCII, to
 facilitate user recognition.
 object-identifier ::= oid-comp object-identifier
 | oid-comp
 oid-comp ::= [ key-string ] "(" numericstring ")"
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 An example representation of an object identifier is:
 joint-iso-ccitt(2) mhs (6) ipms (1) ep (11) ia5-text (0)
 or
 (2) (6) (1)(11)(0)
 Because of the use of brackets and the conflict with the RFC 822
 comment convention, MIXER is defines so that the EBNFobject-
 identifier definition is not used in structured fields.
3.4. Encoding ASCII in Printable String
 Some information in RFC 822 is represented in ASCII, and needs to be
 mapped into X.400 elements encoded as printable string. For this
 reason, a mechanism to represent ASCII encoded as PrintableString is
 needed.
 A structured subset of EBNF.printablestring is now defined. This
 shall be used to encode ASCII in the PrintableString character set.
 ps-encoded = *( ps-restricted-char / ps-encoded-char )
 ps-encoded-char = "(a)" ; (@)
 / "(p)" ; (%)
 / "(b)" ; (!)
 / "(q)" ; (")
 / "(u)" ; (_)
 / "(l)" ; "("
 / "(r)" ; ")"
 / "(" 3DIGIT ")"
 The 822.3DIGIT in EBNF.ps-encoded-char shall have range 0-127, and is
 interpreted in decimal as the corresponding ASCII character. Special
 encodings are given for: at sign (@), percent (%), exclamation
 mark/bang (!), double quote ("), underscore (_), left bracket ((),
 and right bracket ()). These characters, with the exception of round
 brackets, are not included in PrintableString, but are common in RFC
 822 addresses. The abbreviations will ease specification of RFC 822
 addresses from an X.400 system. These special encodings shall be
 interpreted in a case insensitive manner, but always generated in
 lower case.
 A reversible mapping between PrintableString and ASCII can now be
 defined. The reversibility means that some values of printable
 string (containing round braces) cannot be generated from ASCII.
 Therefore, this mapping shall only be used in cases where the
 printable strings have been derived from ASCII (and will therefore
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RFC 2156 MIXER January 1998
 have a restricted domain). For example, in this specification, it is
 only applied to a Domain Defined Attribute which will have been
 generated by use of this specification and a value such as "(" would
 not be possible.
 To encode ASCII as PrintableString, the EBNF.ps-encoded syntax is
 used, with all EBNF.ps-restricted-char mapped directly. All other
 822.CHAR are encoded as EBNF.ps-encoded-char.
 To encode PrintableString as ASCII, parse PrintableString as
 EBNF.ps-encoded, and then reverse the previous mapping. If the
 PrintableString cannot be parsed, then the mapping is being applied
 in to an inappropriate value, and an error shall be given to the
 procedure doing the mapping. In some cases, it may be preferable to
 pass the printable string through unaltered.
 Some examples are now given. Note the arrows which indicate
 asymmetrical mappings:
 PrintableString ASCII
 'a demo.' <-> 'a demo.'
 foo(a)bar <-> foo@bar
 (q)(u)(p)(q) <-> "_%"
 (a) <-> @
 (A) -> @
 (l)a(r) <-> (a)
 (126) <-> ~
 ( -> (
 (l) <-> (
3.5. RFC 1522
 RFC 1522 defines a mechanism for encoding other character set
 information into elements of RFC 822 Headers. A gateway may ignore
 this encoding and treat the elements as ASCII.
 A preferred approach is for the gateway to interpret the RFC 1522
 encoding. This will not always be straightforward, because:
 1. RFC 1522 permits an openly extensible character set choice,
 which may be broader than T.61.
 2. It is not always possible to map all characters into the
 equivalent X.400 field.
 RFC 1522 is only applied to fields which are "for information only".
 A gateway which interprets header elements according to RFC 1522 may
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RFC 2156 MIXER January 1998
 apply reasonable heuristics to minimise information loss.
Chapter 4 - Addressing and Message IDs
 Addressing is the most complex aspect of X.400 <-> RFC 822 gateway
 and is therefore given a separate chapter. This chapter also
 discusses message identifiers, as they are closely linked to
 addresses. This chapter, as a side effect, also defines a textual
 representation of an X.400 OR Address. This specification has much
 similarity to the X.400(92) representation of addresses. This was
 because early versions of this specification were a major input to
 this work. This specification retains compatibility with earlier
 versions. The X.400 specification of address representation can be
 parsed but is not generated.
 Initially we consider an address in the (human) mail user sense of
 "what is typed at the mailsystem to reference a mail user". A basic
 RFC 822 address is defined by the EBNF EBNF.822-address:
 822-address = [ route ] addr-spec
 These definitions are taken from RFC 822. In SMTP (or another 822-
 MTS protocol), the originator and each recipient are considered to be
 defined by such a construct. In an RFC 822 header, the EBNF.822-
 address is encapsulated in the 822-address syntax rule, and there may
 also be associated comments. None of this extra information has any
 semantics, other than to the end user.
 The basic X.400 OR Address, used by the MTS for routing, is defined
 by MTS.ORAddress. In IPMS, the MTS.ORAddress is encapsulated within
 IPMS.ORDescriptor.
 The RFC 822 822.address is mapped with IPMS.ORDescriptor, and that
 RFC 822 EBNF.822-address is mapped with MTS.ORAddress.
 Section 4.1 defines a textual representation of an OR Address, which
 is used throughout the rest of this specification. This text
 representation is designed to represent an X.400 address in the LHS
 (left hand side) or local part of an RFC 822 address, and so this
 representation gives a mechanism to represent X.400 addresses within
 RFC 822 addresses.
 Section 4.2 describes global equivalence mapping between parts of the
 X.400 and RFC 822 name spaces, and defines the concept of a MIXER
 Conformant Global Address Mapping (MCGAM). Gateways conforming to
 this specification shall support MCGAMs.
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RFC 2156 MIXER January 1998
 Section 4.3 is the core part of this chapter, and defines the mapping
 mechanism.
4.1. A textual representation of MTS.ORAddress
 MTS.ORAddress is structured as an ordered set of attributes
 (type/value pairs). It is clearly necessary to be able to encode
 this in ASCII for gatewaying purposes. All components shall be
 encoded, in order to guarantee return of error messages, and to
 optimise third party replies.
4.1.1. Basic OR Address Representation
 An OR Address has a number of structured and unstructured attributes.
 For each unstructured attribute, a key and an encoding is specified.
 For structured attributes, the X.400 attribute is mapped onto one or
 more attribute value pairs. For domain defined attributes, each
 element of the sequence will be mapped onto a triple (key and two
 values), with each value having the same encoding. The attributes
 are as follows, with 1984 attributes given in the first part of the
 attribute key table. For each attribute, a reference is given,
 consisting of the relevant sections in X.402 / ISO 10021-2, and the
 extension identifier for 88 only attributes. The attribute key table
 follows:
Attribute (Component) Key Enc Ref Id
84/88 Attributes
MTS.CountryName C P 18.3.3
MTS.AdministrationDomainName ADMD P 18.3.1
MTS.PrivateDomainName PRMD P 18.3.21
MTS.NetworkAddress X121 N 18.3.7
MTS.TerminalIdentifier T-ID P 18.3.23
MTS.OrganizationName O P/T 18.3.9
MTS.OrganizationalUnitNames.value OU P/T 18.3.10
MTS.NumericUserIdentifier UA-ID N 18.3.8
MTS.PersonalName PN P/T 18.3.12
MTS.PersonalName.surname S P/T 18.3.12
MTS.PersonalName.given-name G P/T 18.3.12
MTS.PersonalName.initials I P/T 18.3.12
MTS.PersonalName
 .generation-qualifier GQ P/T 18.3.12
MTS.DomainDefineAttribute.value DD P/T 18.1
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RFC 2156 MIXER January 1998
88 Attributes
MTS.CommonName CN P/T 18.3.2 1
MTS.TeletexCommonName CN P/T 18.3.2 2
MTS.TeletexOrganizationName O P/T 18.3.9 3
MTS.TeletexPersonalName PN P/T 18.3.12 4
MTS.TeletexPersonalName.surname S P/T 18.3.12 4
MTS.TeletexPersonalName.given-name G P/T 18.3.12 4
MTS.TeletexPersonalName.initials I P/T 18.3.12 4
MTS.TeletexPersonalName
 .generation-qualifier GQ P/T 18.3.12 4
MTS.TeletexOrganizationalUnitNames
 .value OU P/T 18.3.10 5
MTS.TeletexDomainDefinedAttribute
 .value DD P/T 18.1 6
MTS.PDSName PD-SERVICE P 18.3.11 7
MTS.PhysicalDeliveryCountryName PD-C P 18.3.13 8
MTS.PostalCode PD-CODE P 18.3.19 9
MTS.PhysicalDeliveryOfficeName PD-OFFICE P/T 18.3.14 10
MTS.PhysicalDeliveryOfficeNumber PD-OFFICE-NUM P/T 18.3.15 11
MTS.ExtensionORAddressComponents PD-EXT-ADDRESS P/T 18.3.4 12
MTS.PhysicalDeliveryPersonName PD-PN P/T 18.3.17 13
MTS.PhysicalDeliveryOrganizationName PD-O P/T 18.3.16 14
MTS.ExtensionPhysicalDelivery
 AddressComponents PD-EXT-DELIVERY P/T 18.3.5 15
MTS.UnformattedPostalAddress PD-ADDRESS UPA 18.3.25 16
MTS.StreetAddress PD-STREET P/T 18.3.22 17
MTS.PostOfficeBoxAddress PD-BOX P/T 18.3.18 18
MTS.PosteRestanteAddress PD-RESTANTE P/T 18.3.20 19
MTS.UniquePostalName PD-UNIQUE P/T 18.3.26 20
MTS.LocalPostalAttributes PD-LOCAL P/T 18.3.6 21
MTS.ExtendedNetworkAddress
 .e163-4-address.number NET-NUM N 18.3.7 22
MTS.ExtendedNetworkAddress
 .e163-4-address.sub-address NET-SUB N 18.3.7 22
MTS.ExtendedNetworkAddress
 .psap-address NET-PSAP X 18.3.7 22
MTS.TerminalType T-TY I 18.3.24 23
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RFC 2156 MIXER January 1998
 The following keys identify different EBNF encodings, which are
 associated with the ASCII representation of MTS.ORAddress.
 Key Encoding
 P printablestring
 N numericstring
 T teletex-string
 P/T teletex-and-or-ps
 UPA upa-string
 I labelled-integer
 X presentation-address
 The EBNF for presentation-address is taken from the specification RFC
 1278 "A String Encoding of Presentation Address" [23].
 In most cases, the EBNF encoding maps directly to the ASN.1 encoding
 of the attribute. There are a few exceptions. In cases where an
 attribute can be encoded as either a PrintableString or NumericString
 (Country, ADMD, PRMD), either form is mapped into the EBNF. When
 generating ASN.1, the NumericString encoding shall be used if the
 string contains digits and only digits.
 There are a number of cases where the P/T (teletex-and-or-ps)
 representation is used. Where the key maps to a single attribute,
 this choice is reflected in the encoding of the attribute (attributes
 10-21). For example:
 /CN=yen*{165}/
 For most of the 1984 attributes and common name, there is a
 printablestring and a teletex variant. This pair of attributes is
 mapped onto the single component here. This will give a clean
 mapping for the common cases where only one form of the name is used.
 If there is teletex attribute or teletex component only, and it
 contains only characters in the printable string character set, it
 shall be represented in the EBNF as if it had been encoded as
 printable string. A single printable string representation shall
 also be done when both forms are present and they have the same
 printable string representation.
 The Unformatted Postal Address has a slightly more complex mapping
 onto a variant of (teletex-and-or-ps), defined as:
 upa-string = [ printable-upa ] [ "*" teletex-string ]
 printable-upa = printablestring *( "|" printablestring )
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RFC 2156 MIXER January 1998
 The optional teletex part is straightforward. There is an (optional)
 sequence of printable strings which are mapped in order. For
 example:
 /PD-ADDRESS=The Dome|The Square|Richmond|England/
 X.400 (1992) has introduced a string representation of OR Addresses
 (see F.401, Annex B). This has specified a number of string keywords
 for attributes. As earlier versions of this specification were an
 input to this work, many of the keywords are the same. To increase
 compatibility, the following alternative values shall be recognised
 when mapping from RFC 822 to X.400. These shall not be generated
 when mapping from X.400 to RFC 822. The following keyword
 alternative table and the subsequent paragraph lists alternative
 keywords.
 Keyword Alternative
 ADMD A
 PRMD P
 GQ Q
 X121 X.121
 UA-ID N-ID
 PD-OFFICE-NUM PD-OFFICE NUMBER
 PD-OFFICE-NUM PD-OFN
 PD-EXT-ADDRESS PD-EA
 PD-EXT-DELIVERY PD-ED
 PD-OFFICE PD-OF
 PD-STREET PD-S
 PD-UNIQUE PD-U
 PD-LOCAL PD-L
 PD-RESTANTE PD-R
 PD-BOX PD-B
 PD-CODE PD-PC
 PD-SERVICE PD-SN
 DD DDA
 NET-NUM E.164
 NET-PSAP PSAP
 PD-ADDRESS PD-A
 When mapping from RFC 822 to X.400, the keywords defined in this
 paragraph shall be recognized. The ordered keywords: OU1, OU2,
 OU3, and OU4, shall be recognised. If these are present, no
 keyword OU shall be present. These will be treated as ordered
 values of OU. PD-A1, PD-A2, PD-A3, PD-A4, PD-A5, PD-A6 shall be
 treated as ordered lines. If present, these will be assembled
 with separating line feeds to form a single physical address. In
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RFC 2156 MIXER January 1998
 this case PD-ADDRESS (or PD-A) shall not be present. Similarly,
 there are ordered keywords for domain defined attributes: DD1,
 DD2, DD3, DD4,
 If ISDN is present, it may be interpreted as an E.163/164
 address, using local heuristics to parse the string. X.400
 defines the key, but does not give an interpretation of the
 value.
 For T-TY (Terminal Type), the X.400 recommended values are
 preferred, but other values are allowed. These values are: tlx
 (3); ttx (4); g3fax (5); g4fax (6); ia5 (7); and vtx (8).
4.1.2. Encoding of Personal Name
 Handling of Personal Name and Teletex Personal Name is a common
 requirement. Therefore MIXER defines an alternative to the
 EBNF.standard-type syntax, which utilises the "human" conventions for
 encoding these components. A syntax is defined, which is designed to
 provide a clean encoding for the common cases of OR Address
 specification where:
 1. There is no generational qualifier
 2. Initials, if present, contain only letters
 3. Given Name, if present, does not contain full stop ("."),
 and is at least two characters long.
 4. Surname does not contain full stop in the first two
 characters.
 5 If Surname is the only component, it does not contain full
 stop.
 The following EBNF is defined:
 encoded-pn = [ given "." ] *( initial "." ) surname
 given = 2*<ps-char not including ".">
 initial = ALPHA
 surname = printablestring
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RFC 2156 MIXER January 1998
 This is used to map from any string containing only printable string
 characters to an OR address personal name. To map from a string to
 OR Address components, parse the string according to the EBNF. The
 given name and surname are assigned directly. All EBNF.initial
 tokens are concatenated without intervening full stops to generate
 the initials component.
 For an OR address which follows the above restrictions, a string is
 derived in the natural manner. In this case, the mapping will be
 reversible.
 For example:
 GivenName = "Marshall"
 Surname = "Rose"
 Maps with "Marshall.Rose"
 Initials = "MT"
 Surname = "Rose"
 Maps with "M.T.Rose"
 GivenName = "Marshall"
 Initials = "MT"
 Surname = "Rose"
 Maps with "Marshall.M.T.Rose"
 Note that X.400 suggests that Initials is used to encode all initials
 except the surname (X.402 section 18.3.12). Therefore, the defined
 encoding is "natural" when either GivenName or Initials, but not
 both, are present. The case where both are present can be encoded.
4.1.3. Standard Encoding of MTS.ORAddress
 Given this structure, we can specify an EBNF representation of an OR
 Address. The output format of addresses is defined by EBNF.std-or-
 address. The more flexible input format is defined by EBNF.std-or-
 address-input. The input EBNF has been added subsequent to RFC 1327,
 to reflect the formal incorporation of a number of heuristics. The
 address element separator on input may be "/", ";", or a mixture of
 these. The output format is used in all examples.
 std-or-address = 1*( "/" attribute "=" value ) "/"
 attribute = standard-type
 / "RFC-822"
 / dd-key "." std-printablestring
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RFC 2156 MIXER January 1998
 std-or-address-input = [ sep pair ] sep pair *( sep pair )
 sep [ pair sep ]
 sep = "/" / ";"
 pair = input-attribute "=" value
 input-attribute = attribute
 / dd-key ":" std-printablestring
 standard-type = key-string
 dd-key = key-string
 value = std-printablestring
 std-printablestring
 = *( std-char / std-pair )
 std-char = <"{", "}", "*", and any ps-char
 except "/" and "=" >
 std-pair = "$" ps-char
 For address generation, the standard-type is any key defined in the
 key table in Section 4.1, except PN, and DD. For address parsing,
 other key values from Section 4.1 are also valid. The EBNF leads to
 a set of attribute/value pairs. The value is interpreted according to
 the EBNF encoding defined in the table.
 If the standard-type is PN, the value is interpreted according to
 EBNF.encoded-pn, and the components of MTS.PersonalName and/or
 MTS.TeletexPersonalName derived accordingly.
 If dd-key is the recognised Domain Defined string (DD) or one of the
 alternatives defined in Section 4.1, then the type and value are
 interpreted according to the syntax implied from the encoding, and
 aligned to either the teletex or printable string form. Key and
 value shall have the same encoding.
 If value is "RFC-822", then the (printable string) Domain Defined
 Type of "RFC-822" is assumed. This is an optimised encoding of the
 domain defined type defined by this specification.
 The matching of all keywords shall be done in a case-independent
 manner.
 EBNF.std-or-address uses the characters "/" and "=" as delimiters.
 Domain Defined Attributes and any value may contain these characters.
 A quoting mechanism, using the non-printable string "$" is used to
 allow these characters to be represented.
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RFC 2156 MIXER January 1998
 If an address of this syntax is parsed, and a country value is
 present, but no ADMD, the string shall be interpreted as if an ADMD
 value of single space had been specified.
4.2. Global Address Mapping
 From a user perspective, the ideal mapping would be entirely
 symmetrical and global, to enable addresses to be referred to
 transparently in the remote system, with the choice of gateway being
 left to the Message Transfer Service. There are two fundamental
 reasons why this is not possible:
 1. The syntaxes are sufficiently different to make this
 impossible.
 2 There is insufficient administrative co-operation between
 the X.400 and RFC 822 name registration authorities for this
 to work.
 Another way to view this situation is to see that there is not a full
 global equivalence between X.400 and RFC 822 addressing. To meet
 user needs to the extent possible, this specification provides for
 equivalence where there is sufficient co-operation. To be useful,
 this equivalence shall be recognised and interpreted in the same way
 by all gateways. Therefore, an asymmetrical mapping is defined,
 which can be symmetrical where there is appropriate administrative
 co-operation. Section 4.3 describes the asymetrical aspects. This
 section describes a mechanism to enable the administrative co-
 ordination for symmetrical mappings.
 In order to achieve a symmetrical mapping there is a need to define
 an administrative equivalence between parts of the OR Address and
 Domain namespaces. Previous version of this specification did this
 by definition of a global set of mappings. MIXER defines the concept
 of a MIXER Conformant Global Address Mapping (MCGAM). This acronym
 is defined so that it is very clear what is being referenced.
 The X.400 and Internet Mail address spaces are hierarchical. It is
 possible to define an equivalence between two points in the
 hierarchies, such that addresses below that point can be derived in
 an algorithmic manner. An MCGAM is a mapping from a point in one
 hierarchy to a point in the other hierarchy. An "MGGAM pair" is a
 pair of symmetrical mappings between two points. To define an MCGAM,
 the following shall apply:
 1. The authority defining the MCGAM shall have responsibility
 for BOTH of the namespaces between which the MCGAM is
 defined.
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RFC 2156 MIXER January 1998
 2. The authority defining the MCGAM is responsible to ensure
 that addresses allocated below the two equivalence points
 conform to the rules set out below.
 3. The authority defining the MCGAM is responsible to ensure
 that addresses which are generated according to the MCGAM
 are routed correctly.
 In general, MCGAMs will be independent. In some cases, a set of
 MCGAMs may be related (e.g., where one MCGAM defines a mapping for an
 organization and a second MCGAM defines an excpetion for a subtree
 within the organization). In this case, the related set of MCGAMs
 shall be treated as a single MCGAM for distribution purposes.
 The existence of an MCGAM does not imply routability and access for
 all users.
 The authority defining an MCGAM may simply use this mapping locally.
 This will often be the case in a "local scenario" gateway. Because
 of third party addressing, a MIXER gateway will work best with the
 maximum number of MCGAMs. Therefore, three mechanisms are defined
 to enable publication and exchange of MCGAMs:
 1. Distribution of text tables. This is described in Appendix
 F of this specification.
 2. Distribution by Domain Name Service. This is described in
 RFC 2163 [3].
 3. Distribution by X.500 Directory Service. This is defined
 in RFC 2164 [26].
 The following sections define how the MCGAM namespace equivalence is
 modelled. The Internet Domain Namespace defines a simple hierarchy.
 For the purposes of this mapping, only parts of the namespace where
 domains conform to the EBNF domain-syntax are allowed.
 domain-syntax = alphanum [ *alphanumhyphen alphanum ]
 alphanum = <ALPHA or DIGIT>
 alphanumhyphen = <ALPHA or DIGIT or HYPHEN>
 Although RFC 822 allows for a more general syntax, this restricted
 syntax is used in MIXER as it is the one chosen by the various domain
 service administrations. In practice, it reflects all RFC 822 usage.
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RFC 2156 MIXER January 1998
 The following OR Address attributes are considered as a hierarchy,
 and may be specified by the domain. They are (in order of the
 hierarchy defined by MIXER):
 Country, ADMD, PRMD, Organization, Organizational Units
 There may be up to four ordered Organizational Units. This
 hierarchy reflects most usage of X.400, although X.400 may be used in
 other ways. In particular, it covers the Mnemonic OR Address using a
 1984 compatible encoding. This is seen as the dominant form of OR
 Address. MCGAMs may only be used when this hierarchy applies.
 An equivalence mapping is defined between two nodes in the respective
 hierarchies. For example:
 => "AC.UK" might be mapped with
 PRMD="UK.AC", ADMD="GOLD 400", C="GB"
 The mapping identifies that the management of these points in the
 respective hierarchies is the same (or co-operate very closely). The
 equivalence means that the namespaces below this equivalence point
 map 1:1, except where the mapping is overridden by further
 equivalence mappings lower down the hierarchy. This equivalence may
 be achieved in three ways:
 1. All of the nodes below this point are RFC 822, and the MIXER
 mapping defines the X.400 addresses for these nodes.
 2. All of the nodes below this point are X.400, and the MIXER
 mapping defines the RFC 822 addresses for these nodes.
 3. There are X.400 and RFC 822 nodes below this point, and
 addressing is managed in a manner which ensures the
 equivalence. The rules to achieve this are defined by
 MIXER.
 Each of these ways gives a framework for MCGAM definition.
 When an MCGAM is defined, a systematic mapping for the inferior nodes
 in the two hierarchies follows. This is a 1:1 mapping between the
 nodes in the subtrees. For example, given the MCGAM pair defined
 above:
 the domain "R-D.Salford.AC.UK" algorithmically maps with
 OU="R-D", O="Salford", PRMD="UK.AC", ADMD="GOLD 400", C="GB"
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RFC 2156 MIXER January 1998
 Note that when an equivalence is defined, that this can be re-defined
 for lower points in the hierarchy. However, it is not possible to
 declare contained subtrees to be un-mappable.
 The equivalence mapping also provides a mechanism to deal with
 missing elements in the X.400 hierarchy (most commonly the PRMD,
 which is the only element that may be ommitted when conforming to
 recent versions of X.400). A domain may be associated with an
 omitted attribute in conjunction with several present ones. When
 performing the algorithmic insertion of components lower in the
 hierarchy, the omitted value shall be skipped. For example:
 If there is an MCGAM pair between domain HNE.EGM" and "O=HNE",
 "ADMD=ECQ", "C=TC", and omitted PRMD
 then
 "ZI.HNE.EGM" is algorithmically mapped with "OU=I", "O=HNE",
 "ADMD=ECQ", "C=TC"
 Attributes may have null values, and this is treated separately from
 omitted attributes (while it is not ideal to make this distinction,
 it is useful in practice).
4.2.1. Directory and Nameserver Mappings
 When a set of MCGAMs are supported by X.500 or DNS, there is the
 possibility that results will be indeterminate due to timeout.
 Lookup shall be repeated until a value is determined, in order to
 maintain consistent gateway operation.
 Where the mapping relates to an envelope address, the gateway shall
 non-deliver messages according to the associated MTA's normal timeout
 policy. Where the mapping relates to addresses in the message
 header, there shall be a timeout in the range of 1-4 hours or shorter
 if this is required to maintain quality of service constraints. If
 a mapping cannot be done in this time, address encapsulation shall be
 used.
4.3. EBNF.822-address <-> MTS.ORAddress
 This section defines the basic address mapping.
4.3.1. X.400 encoded in RFC 822
 This section defines how X.400 addresses are represented in RFC 822
 addresses.
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RFC 2156 MIXER January 1998
 The std-or-address syntax is used to encode OR Address information
 in the 822.local-part of EBNF.822-address. Where there is an
 applicable equivalence mapping, further OR Address information is
 associated with the 822.domain component. This cannot be used in the
 general case, due to character set problems, and to the variants of
 X.400 OR Addresses which use different attribute types. The only way
 to encode the full PrintableString character set in a domain is by
 use of the 822.domain-ref syntax (i.e. 822.atom). This is likely to
 cause problems on many systems. The effective character set of
 domains is in practice reduced from the RFC 822 set, by restrictions
 imposed by domain conventions and policy [10], and by the EBNF
 definition in SMTP.
 A generic 822.address consists of a 822.local-part and a sequence of
 822.domains (e.g., <@domain1,@domain2:user@domain3>). All except the
 822.domain associated with the 822.local-part (domain3 in this case)
 are considered to specify routing within the RFC 822 world, and will
 not be interpreted by the gateway (although they may have identified
 the gateway from within the RFC 822 world).
 The 822.domain associated with the 822.local-part identifies the
 gateway from within the RFC 822 world. This final 822.domain may be
 used to determine some number of OR Address attributes, where this
 does not conflict with the first role. RFC 822 routing to gateways
 will usually be set up to facilitate the 822.domain being used for
 both purposes.
 In the case that there is no applicable equivalence mapping, all of
 the X.400 address is encoded in the 822.local-part and the 822.domain
 identifies the gateway to which the message is being sent. This
 technique may be used by the RFC 822 user for any X.400 address where
 the equivalence mapping is not known.
 In the case that there is an applicable MCGAM, the maximum number of
 attributes are encoded in the 822.domain. The remaining attributes
 are encoded on the LHS, using the EBNF.std-or-address syntax. For
 example:
 /I=J/S=Linnimouth/GQ=5/@Marketing.Widget.COM
 encodes the MTS.ORAddress consisting of:
 MTS.CountryName = "TC"
 MTS.AdministrationDomainName = "BTT"
 MTS.OrganizationName = "Widget"
 MTS.OrganizationalUnitNames.value = "Marketing"
 MTS.PersonalName.surname = "Linnimouth"
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RFC 2156 MIXER January 1998
 MTS.PersonalName.initials = "J"
 MTS.PersonalName.generation-qualifier = "5"
 on the basis of an MCGAM pair between:
 Domain: Widget.COM
 OR Address: O="Widget", ADMD="BTT", C="TC"
 Given the OR address, the domain Widget.COM is determined from the
 equivalence mapping and the next component is determined
 algorithmically to give Marketing.Widget.COM. The remaining
 attributes are encoded on the LHS in 822.local-part.
 There is a further mechanism to simplify the encoding of common
 cases, where the only attributes to be encoded on the LHS are (non-
 Teletex) Personal Name attributes which comply with the restrictions
 of 4.1.2. To achieve this, the 822.local-part shall be encoded as
 EBNF.encoded-pn. In the previous example, if the GenerationQualifier
 was not present in the OR Address, it would map with the RFC 822
 address: J.Linnimouth@Marketing.Widget.COM.
 From the standpoint of the RFC 822 Message Transfer System, the
 domain specification is used to route the message in the standard
 manner. The standard domain mechanisms are used to select
 appropriate gateways for the corresponding OR Address space. It is
 the responsibility of the management that defines the equivalence
 mapping to define routing in the manner which will enable the message
 to be delivered.
4.3.2. RFC 822 encoded in X.400
 The previous section showed a mapping from X.400 to RFC 822. In the
 case where the mapping was symmetrical and based on the equivalence
 mapping, this has also shown how RFC 822 is encoded in the X.400.
 This equivalence cannot be used for all RFC 822 addresses.
 The general case is mapped by use of domain defined attributes. A
 (Printable String) Domain defined type "RFC-822" is defined. The
 associated attribute value is an ASCII string encoded according to
 Section 3.3.3 of this specification. The interpretation of the ASCII
 string follows RFC 822, and RFC 1123 [10,16]. Domains shall always
 be fully qualified.
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RFC 2156 MIXER January 1998
 Other OR Address attributes will be used to identify a context in
 which the OR Address will be interpreted. This might be a Management
 Domain, or some part of a Management Domain which identifies a
 gateway MTA. For example:
 C = "GB"
 ADMD = "GOLD 400"
 PRMD = "UK.AC"
 O = "UCL"
 OU = "CS"
 "RFC-822" = "Jimmy(a)WIDGET-LABS.CO.UK"
 OR
 C = "TC"
 ADMD = "Wizz.mail"
 PRMD = "42"
 "rfc-822" = "postel(a)venera.isi.edu"
 Note in each case the PrintableString encoding of "@" as "(a)". In
 the second example, the "RFC-822" domain defined attribute is
 interpreted everywhere within the (Private) Management Domain. In
 the first example, further attributes are needed within the
 Management Domain to identify a gateway. Thus, this scheme can be
 used with varying levels of Management Domain co-operation.
 There is a limit of 128 characters in the length of value of a domain
 defined attribute, and an OR Address can have a maxmimum of four
 domain defined attributes. Where the printable string generated from
 the RFC 822 address exceeds 128 characters, additional domain defined
 attributes are used to enable up to 512 characters to be encoded.
 These attributes shall be filled completely before the next one is
 started. The (Printable String) DDA keywords are: RFC822C1;
 RFC822C2; RFC822C3. Longer addresses cannot be encoded.
 MIXER defines a representation of RFC 822 addresses in printable
 string domain defined attributes. Teletex domain defined attributes
 with a key of RFC-822, RFC822C1; RFC822C2; RFC822C3 shall not be
 generated. This is for backwards compatibility reasons.
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RFC 2156 MIXER January 1998
 Reception of these attributes in the manner defined below is
 mandatory. This is to allow the possibility for future versions of
 MIXER to allow generation of teletex domain defined attributes.
 Where the values of all of these teletex domain defined attributes
 are printable string characters, they shall be interpreted in the
 same way as the printable string domain defined attributes. If this
 is not the case, the printable string encoding translation shall be
 omitted. If both teletex and printable string attributes are
 present, this is valid if and only if they represent exactly the same
 RFC 822 address.
4.3.3. Component Ordering
 In most cases, ordering of OR Address components is not significant
 for the mappings specified. However, Organizational Units (printable
 string and teletex forms) and Domain Defined Attributes are specified
 as SEQUENCE in MTS.ORAddress, and so their order may be significant.
 This specification needs to take account of this:
 1. To allow consistent mapping into the domain hierarchy
 2. To ensure preservation of order over multiple mappings.
 There are three places where an order is specified:
 1. The text encoding (std-or-address) of MTS.ORAddress as used
 in the local-part of an RFC 822 address. An order is needed
 for those components which may have multiple values
 (Organizational Unit, and Domain Defined Attributes). When
 generating an 822.std-or-address, components of a given type
 shall be in hierarchical order with the most significant
 component on the RHS (right hand side or domain part). If
 there is an Organization Attribute, it shall be to the right
 of any Organizational Unit attributes. These requirements
 are for the following reasons:
 - Alignment to the hierarchy of other components in RFC
 822 addresses (thus, Organizational Units will appear
 in the same order, whether encoded on the RHS or LHS).
 - Backwards compatibility with RFC 987/1026.
 - To ensure that gateways generate consistent addresses.
 This is both to help end users, and to generate
 identical message ids.
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 Further, it is recommended that all other attributes are generated
 according to this ordering, so that all attributes so encoded follow
 a consistent hierarchy. When generating 822.msg-id, this order shall
 be followed.
 2. For the Organizational Units (OU) in MTS.ORAddress, the
 first OU in the SEQUENCE is the most significant, as specified
 in X.400.
 3. For the Domain Defined Attributes in MTS.ORAddress, the
 First Domain Defined Attribute in the SEQUENCE is the most
 significant.
 Note that although this ordering is mandatory for this mapping, MIXER
 does not give additional implications on the ordering significance
 within X.400.
4.3.4. RFC 822 -> X.400 Basic Address Mapping
 There are two basic cases:
 1. X.400 addresses encoded in RFC 822. This will also include
 RFC 822 addresses which are given reversible encodings.
 2. "Genuine" RFC 822 addresses.
 The mapping shall proceed as follows, by first assuming case 1).
 STAGE I.
 1. If the 822-address is not of the form:
 local-part "@" domain
 take the domain which will be routed on and apply step 2 of stage
 1 to derive (a possibly null) set of attributes. Then go to stage
 II.
 The gateway may reduce a source route address to this form by
 removal of all but the last domain. In terms of the design
 intentions of RFC 822, this would be an incorrect action. (Note
 that an address of the form local%part@domain is not a source
 route). However, in most cases, it will provide a better service
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 to the end user, and is in line with the Internet Host
 Requirements. This is a reflection on the common inappropriate
 use of source routing in RFC 822 based systems, despite the
 discussion in the Host Requirements [10]. Either approach, or
 the intermediate approach of stripping only domain references
 which reference the local gateway are conformant to this
 specification.
 2. If the 822.local-part uses the 822.quoted-string encoding,
 remove this quoting. If the resulting unquoted
 822.local-part has leading space, trailing space, or two
 adjacent spaces go to stage II.
 3. If the unquoted 822.local-part contains any characters not
 in PrintableString, "{", "}", "*", and "$", go to stage II.
 4. Parse the (unquoted) 822.local-part according to the EBNF
 EBNF.std-or-address-input. Checking of upper bounds shall
 not be done at this point. If this parse fails, parse the
 local-part according to the EBNF EBNF.encoded-pn. If this
 parse fails, go to stage II. The result is a set of
 type/value pairs.
 5. Associate the EBNF.attribute-value syntax (determined from
 the identified type) with each value, and check that it
 conforms. If not, go to stage II.
 6. If the set of attributes forms a valid X.400 address,
 according to X.402, then go to step 9. All forms of X.400
 address are allowed at this stage. Steps 7-8 default
 attributes for certain types of OR Address.
 7. If the set of attributes cannot form a mnemonic form of
 X.400 address after addition of attributes which may be
 derived from the EBNF.domain (C, ADMD, PRMD, O, OU), go to
 stage II.
 8. Attempt to parse EBNF.domain as:
 *( domain-syntax "." ) known-domain
 Where EBNF.known-domain is the longest possible match in the set
 of MCGAMs being used by the gateway (described in Section 4.2).
 EBNF.domain-syntax is the restricted domain syntax defined in
 Section 4.2, to which all of the domain components shall conform
 for the parse to be successful. If this fails, go to stage II.
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 For each component, systematically allocate the attribute
 implied by each EBNF.domain-syntax component in the order: C,
 ADMD, PRMD, O, OU. Note that if the MCGAM used identifies an
 "omitted attribute", then this attribute shall be omitted in the
 systematic allocation. If this new component exceed an upper
 bound (ADMD: 16; PRMD: 16; O: 64; OU: 32) or it would lead to
 more than four OUs, then go to stage II with the attributes
 derived.
 The attributes derived in this step (referred to as RHS
 attributes) are merged with the ones derived from the LHS (step
 6). In some cases, not all of the RHF attributes are used. LHS
 attributes are all used. C will not be in the LHS attributes.
 If ADMD is in the LHS attributes, only C is taken from the RHS
 attributes. If PRMD is in the LHS attributes, C and ADMD are
 taken from the RHS attributes. If O is on the LHS, C, ADMD and
 PRMD (if present) are taken from the RHS attributes. In other
 cases all RHS attributes are taken.
 9. Ensure that the set of attributes conforms both to the
 MTS.ORAddress specification and to the restrictions on this
 set given in X.400, and that no upper bounds are exceeded
 for any attribute. If not go to stage II.
 10. Build the OR Address from this information.
 STAGE II.
 This will only be reached if the RFC 822 EBNF.822-address is not a
 valid X.400 encoding. This implies that the address refers to a
 recipient on an RFC 822 system or that the encoding of the address is
 invalid. Such addresses shall be encoded in an X.400 OR Address
 using a domain defined attribute.
 1. Convert the EBNF.822-address to PrintableString, as
 specified in Chapter 3.
 2. Generate the "RFC-822" domain defined attribute from this
 string.
 3. Build the rest of the OR Address in the manner described
 below.
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 It is not always possible to encode the domain defined attribute
 due to length restrictions. If the limit is exceeded by a
 mapping at the MTS level, then the gateway shall reject the
 message in question. If this occurs at the IPMS level, then the
 action will depend on the policy being taken for IPMS encoding,
 which is discussed in Section 5.1.3.
 Use Stage I, step 8, to generate a set of attributes to build the
 remainder of the address. The administrative equivalence of the
 mappings will ensure correct routing through X.400 to a gateway
 back to RFC 822.
 If Stage I, step 8 does not generate a set of attributes or
 the address generated is unroutable, the remained of the OR
 address is generated as follows. The remainder of the OR address
 effectively identifies a source route to a gateway from the X.400
 side. There are three cases, which are handled differently:
 SMTP Return Address
 This shall be set up so that errors are returned through the
 same gateway. Therefore, the OR Address of the local
 gateway shall be used.
 IPMS Addresses
 These are optimised for replying. In general, the message
 may end up anywhere within the X.400 world, and so this
 optimisation identifies a gateway appropriate for the RFC
 822 address being converted. The 822.domain to which the
 address would be routed is used to select an appropriate
 gateway.
 In this case, it may be useful to use a non-local gateway,
 which will optimise the reply address. This information
 may be looked up in gateway tables in a manner equivalent to
 the MCGAM lookup. Because of the similarity of lookup, the
 three MCGAM lookup mechanisms (table, X.500, DNS) are also
 available to look up this information. This information is
 local, and a gateway may insert any appropriate (gateway)
 OR Address. The longest possible match on the 822.domain
 defines which gateway to use. This mechanism is used for
 any part of the X.400 namespace for which it is desirable to
 identify a preferred X.400 gateway in order to optimise
 routing.
 If no mapping is found for the 822.domain, a default value
 (typically that of the local gateway) is used. It is never
 appropriate to ignore the locally used MCGAMs.
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RFC 2156 MIXER January 1998
 SMTP Recipient
 As the RFC 822 and X.400 worlds are in principle fully
 connected, there is no technical reason for this situation
 to occur. In practice, this is not the case. In some cases,
 routing may be configured to use X.400 to connect an RFC 822
 island to the Internet. The information that this part of
 the domain space is to be routed by X.400 rather than
 remaining within the RFC 822 world shall be configured
 privately into the gateway in question. X.400 routing shall
 not make use of the presence of the RFC-822 DDA to perform
 X.400 routing. The OR address shall then be generated in
 the same manner as for an IPMS address, using the locally
 available MCGAMs. It is to support this case that the
 definition of the global domain to gateway mapping is
 important, as the use of this mapping will lead to a remote
 X.400 address, which can be routed by X.400 routing
 procedures. The information in this mapping shall not be
 used as a basis for deciding to convert a message from RFC
 822 to X.400.
 Three examples are given, neither of which has applicable MCGAMs.
 Example 1: (Address not in "localpart" "@" "domainpart")
 @relay.co.uk:userb@host2
 maps to
 c=gb; a= ; p=uk.ac; o=mr; dd.rfc-822=(a)relay.co.uk:userb(a)host2;
 Example 2: (Address with non printablestring characters)
 Tom_Harris@cs.widget.com
 maps to
 c=us; a=MCI; P=relay; dd.rfc-822=Tom(u)Harris(a)cs.widget.com;
 Example 3: (Address with an entry for alter.net into the OR Address
 of Preferred Gateway table, pointing to c=gb; A=BTglobal; P=relay)
 postmaster@UK.alter.net
 maps to
 c=gb; a=BTglobal; P=relay; dd.rfc-822=postmaster(a)UK.alter.net;
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4.3.4.1. Heuristic for mapping RFC 822 to X.400
 The following heuristic, which relates to ordering of address
 components, may be used when mapping from RFC 822 to X.400. The
 ordering of attributes may be inverted or mixed, and so the following
 heuristics may be applied:
 If there is an Organization attribute to the left of any Org Unit
 attribute, assume that the hierarchy is inverted. This is to
 facilitate the situation where a user has input the attributes in
 reverse hierarchical order. To do this the gateway shall first
 map according to the order defined in 4.3.3. If this mapping
 generates an address which X.400 address verification shows to be
 invalid, this heuristic may be applied as an alternative to
 immediate rejection of the address.
4.3.5. X.400 -> RFC 822 Basic Address Mapping
 There are two basic cases:
 1. RFC 822 addresses encoded in X.400.
 2. "Genuine" X.400 addresses. This may include symmetrically
 encoded RFC 822 addresses.
 When an MTS Recipient OR Address is interpreted, gatewaying will be
 selected if there is a single "RFC-822" domain defined attribute
 present. In this case, use mapping A and in other cases, use mapping
 B.
 RFC 1327 specified that this shall only be done when the gateway
 identfied is local or otherwise known, and identified the approach
 specified here as a pragmatic option. Experience has shown that this
 is effective in practice, despite theoretical problems.
 If a gateway wishes to make a mapping in a manner similar to RFC
 1327, but does not wish for this global interpretation (e.g., to
 support an RFC 822 local system, which does not use global
 addressing), then it may choose a private domain defined attribute,
 different to "RFC-822". An RFC 1327 gateway might be configurable to
 operate in this manner.
 Mapping A
 1. Map the domain defined attribute value to ASCII, as defined
 in Chapter 3, and drop all other attributes.
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RFC 2156 MIXER January 1998
 Mapping B
 This is used for X.400 addresses which do not use the explicit RFC
 822 encoding.
 1. For all string encoded attributes, remove any leading or
 trailing spaces, and replace adjacent spaces with a single
 space.
 The only attribute which is permitted to have zero length is
 the ADMD. This shall be mapped onto a single space.
 These transformations are for lookup only. If an
 EBNF.std-or-address mapping is used as in 4), then the
 original values shall be used.
 2. The numeric country codes may be mapped to the two letter
 values (as defined in ISO 3166). Global mappings are
 usually only defined in terms of the ISO 3166 codes.
 3. Noting the hierarchy specified in 4.3.1 and including
 omitted attributes, determine the maximum set of attributes
 which have an associated domain specification in the local
 set of MCGAMs. If no match is found, allocate the domain as
 described below, and go to step 5. The default domain to be
 used is the specification of the local gateway. A gateway
 may use other domains according to private mapping tables or
 heuristics. For example, it may choose a domain which it
 knows to provide a free gateway service to the mapped
 address.
 In cases where the address refers to an X.400 UA, it is
 important that the generated domain will correctly route to
 a gateway. In general, this is achieved by carefully co-
 ordinating RFC 822 routing with the definition of the
 MCGAMs, as there is no easy way for the gateway to make this
 check. One rule that shall be used is that domains with
 only one component will not route to a gateway. If the
 generated domain does not route correctly, the address is
 treated as if no match is found.
 The gateway may also make use of a mapping equivalent to the
 MCGAM mapping to determine the domain to use. This mapping
 is done from the OR Address hierarchy. This is not a
 global mapping, but is a routing style mapping from the OR
 Address space, to enable a best choice domain to be
 inserted. This mapping is supported by the three MCGAM
 lookup mechanisms.
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RFC 2156 MIXER January 1998
 4. The mapping identified in 3) gives a domain, and an OR
 address prefix. Follow the hierarchy: C, ADMD, PRMD, O, OU.
 For each successive component below the OR address prefix, which
 conforms to the syntax EBNF.domain-syntax (as defined in 4.3.1),
 allocate the next subdomain. At least one attribute of the
 X.400 address shall not be mapped onto subdomain, as 822.local-
 part cannot be null. If there are omitted attributes in the OR
 address prefix, these will have correctly and uniquely mapped to
 a domain component. Where there is an attribute omitted below
 the prefix, all attributes remaining in the OR address shall be
 encoded on the LHS. This is to ensure a reversible mapping. For
 example, if there is an address /S=XX/O=YY/ADMD=A/C=NN/ and a
 mapping for /ADMD=A/C=NN/ is used, then /S=XX/O=YY/ is encoded
 on the LHS.
 5. If the address contains any attribute not used in mnemonic
 form, then all of the attributes in the address shall be encoded
 on the LHS in EBNF.std-or-address syntax, as described below.
 For addresses of mnemonic form, if the remaining components are
 personal-name components, conforming to the restrictions of
 4.2.1, then EBNF.encoded-pn is derived to form 822.local-part.
 In other cases the remaining components are simply encoded as
 822.local-part using the EBNF.std-or-address syntax. If
 necessary, the 822.quoted-string encoding is used. The
 following are examples of legal quoting: "a b".c@x; "a b.c"@x.
 Either form may be generated. Generation of the latter style is
 strongly recommended.
 Four examples are given.
 Example 1: (Address with missing X.400 elements and no specific
 mapping rule for "o=sales; a=Master400; C=it", where a mapping exists
 for a=master400; C=it;)
 S=Support; O=sales; A=Master400; C=it;
 maps to
 /S=Support/o=sales/@Master400.it
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 Example 2: (Address with illegal characters in RFC822 generated
 domain if default hierarchical translation (specific mapping rule is
 existing for c=fr; a=atlas; p=autoroutes) is used)
 S=renseignements; O=Region Parisienne; P=autoroutes; A=atlas; C=fr;
 maps to
 "/S=renseignements/o=Region Parisienne/"@autoroutes.fr
 Example 3: (Address containing elements not mappable into RFC822
 local part)
 S=Rossi; DD.cap=20100; DD.ph1=Via Larga 11; DDA.city=Milano;
 A=PtPostel; C=it;
 maps to
 "/DD.cap=20100/DD.ph1=Via Larga
 11/DD.city=Milano/S=Rossi/"@ptpostel.it
 Example 4: (Address with an entry for A=ATT; C=us; into the domain
 of Preferred Gateway table, pointing to attmail.com)
 G=Andy; S=Wharol; O=MMNY; A=ATT; C=us;
 maps to
 /G=Andy/S=Wharol/O=MMNY@attmail.com
4.4. Repeated Mappings
 There are two types of repeated mapping:
 1. A recursive mapping, where the repeat is within one gateway
 2 A source route, where the repetition occurs across multiple
 gateways
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4.4.1. Recursive Mappings
 It is possible to supply an address which is recursive at a single
 gateway. For example:
 C = "XX"
 ADMD = "YY"
 O = "ZZ"
 "RFC-822" = "Smith(a)ZZ.YY.XX"
 This is mapped first to an RFC 822 address, and then back to the
 X.400 address:
 C = "XX"
 ADMD = "YY"
 O = "ZZ"
 Surname = "Smith"
 In some situations this type of recursion may be frequent. It is
 important where this occurs, that no unnecessary protocol conversion
 occurs. This will minimise loss of service.
4.4.2. Source Routes
 The mappings defined are symmetrical and reversible across a single
 gateway. The symmetry is particularly useful in cases of (mail
 exploder type) distribution list expansion. For example, an X.400
 user sends to a list on an RFC 822 system which he belongs to. The
 received message will have the originator and any 3rd party X.400 OR
 Addresses in correct format (rather than doubly encoded). In cases
 (X.400 or RFC 822) where there is common agreement on gateway
 identification, then this will apply to multiple gateways.
 When a message traverses multiple gateways, the mapping will always
 be reversible, in that a reply can be generated which will correctly
 reverse the path. In many cases, the mapping will also be
 symmetrical, which will appear clean to the end user. For example,
 if countries "AB" and "XY" have RFC 822 networks, but are
 interconnected by X.400, the following may happen: The originator
 specifies:
 Joe.Soap@Widget.PTT.XY
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RFC 2156 MIXER January 1998
 This is routed to a gateway, which generates:
 C = "XY"
 ADMD = "PTT"
 PRMD = "Griddle MHS Providers"
 Organization = "Widget Corporation"
 Surname = "Soap"
 Given Name = "Joe"
 This is then routed to another gateway where the mapping is reversed
 to give:
 Joe.Soap@Widget.PTT.XY
 Here, use of the gateway is transparent.
 Mappings will only be symmetrical where mapping equivalences are
 defined. In other cases, the reversibility is more important, due to
 the (far too frequent) cases where RFC 822 and X.400 services are
 partitioned.
 The syntax may be used to source route. THIS IS STRONGLY
 DISCOURAGED. For example:
 X.400 -> RFC 822 -> X.400
 C = "UK"
 ADMD = "Gold 400"
 PRMD = "UK.AC"
 "RFC-822" = "/PN=Duval/DD.Title=Manager/(a)Inria.ATLAS.FR"
 This will be sent to an arbitrary UK Academic Community gateway by
 X.400. Then it will be sent by JNT Mail to another gateway
 determined by the domain Inria.ATLAS.FR (FR.ATLAS.Inria). This will
 then derive the X.400 OR Address:
 C = "FR"
 ADMD = "ATLAS"
 PRMD = "Inria"
 PN.S = "Duval"
 "Title" = "Manager"
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RFC 2156 MIXER January 1998
 Similarly:
 RFC 822 -> X.400 -> RFC 822
 "/RFC-822=jj(a)seismo.css.gov/PRMD=AC/ADMD=BT/C=GB/"@monet.berkeley.edu
 This will be sent to monet.berkeley.edu by RFC 822, then to the
 AC PRMD by X.400, and then to jj@seismo.css.gov by RFC 822.
4.5. Directory Names
 Directory Names are an optional part of OR Name, along with OR
 Address. The RFC 822 addresses are mapped onto the OR Address
 component. As there is no functional mapping for the Directory Name
 on the RFC 822 side, a textual mapping is used. There is no
 requirement for reversibility in terms of the goals of this
 specification. There may be some loss of functionality in terms of
 third party recipients where only a directory name is given, but this
 seems preferable to the significant extra complexity of adding a full
 mapping for Directory Names.
 The Directory Name shall be represented within an RFC 822 comment
 using the comaptible formats of RFC 1484 or RFC 1485. It is
 recommended that the directory string format of RFC 1485 is used
 [24]. The User Friendly Name form of RFC 1484 may be used [25].
4.6. MTS Mappings
 The basic mappings at the MTS level are:
 1) SMTP originator ->
 MTS.PerMessageSubmissionFields.originator-name
 MTS.OtherMessageDeliveryFields.originator-name ->
 SMTP originator
 2) SMTP recipient ->
 MTS.PerRecipientMessageSubmissionFields
 MTS.OtherMessageDeliveryFields.this-recipient-name ->
 SMTP recipient
 SMTP recipients and return addresses are encoded as EBNF.822-address.
 The MTS Originator is always encoded as MTS.OriginatorName, which
 maps onto MTS.ORAddressAndOptionalDirectoryName, which in turn maps
 onto MTS.ORName.
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4.6.1. RFC 822 -> X.400 MTS Mappings
 From the SMTP Originator, use the basic ORAddress mapping, to
 generate MTS.PerMessageSubmissionFields.originator-name (MTS.ORName),
 without a DirectoryName.
 For recipients, the following settings are made for each component of
 MTS.PerRecipientMessageSubmissionFields.
 recipient-name
 This is derived from the SMTP recipient by the basic ORAddress
 mapping.
 originator-report-request
 This may either be set to "delivery-report", or set according to
 SMTP extensions as set out in Appendix A.
 explicit-conversion
 This optional component is omitted, as this service is not needed
 extensions
 The default value (no extensions) is used
4.6.2. X.400 -> RFC 822 MTS Mappings
 The basic functionality is to generate the SMTP originator and
 recipients. There is information present on the X.400 side, which
 cannot be mapped into analogous SMTP services. For this reason, new
 RFC 822 fields are added for the MTS Originator and Recipients. The
 information discarded at the SMTP level will be present in these
 fields. In some cases a (positive) delivery report will be generated.
4.6.2.1. SMTP Mappings
 Use the basic ORAddress mapping, to generate the SMTP originator
 (return address) from MTS.OtherMessageDeliveryFields.originator-name
 (MTS.ORName). If MTS.ORName.directory-name is present, it is
 discarded. (Note that it will be presented to the user, as described
 in 4.6.2.2).
 The mapping uses the MTA level information, and maps each value of
 MTA.PerRecipientMessageTransferFields.recipient-name, where the
 responsibility bit is set, onto an SMTP recipient.
 Note:The SMTP recipient is conceptually generated from
 MTS.OtherMessageDeliveryFields.this-recipient-name. This is done
 by taking MTS.OtherMessageDeliveryFields.this-recipient-name, and
 generating an SMTP recipient according to the basic ORAddress
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RFC 2156 MIXER January 1998
 mapping, discarding MTS.ORName.directory-name if present.
 However, if this model was followed exactly, there would be no
 possibility to have multiple SMTP recipients on a single message.
 This is unacceptable, and so layering is violated.
4.6.2.2. Generation of RFC 822 Headers
 Not all per-recipient information can be passed at the SMTP level.
 For this reason, two new RFC 822 headers are created, in order to
 carry this information to the RFC 822 recipient. These fields are
 "X400-Originator:" and "X400-Recipients:".
 The "X400-Originator:" field is set to the same value as the SMTP
 originator. In addition, if
 MTS.OtherMessageDeliveryFields.originator-name (MTS.ORName) contains
 MTS.ORName.directory-name then this Directory Name shall be
 represented in an 822.comment.
 Recipient names, taken from each value of
 MTS.OtherMessageDeliveryFields.this-recipient-name and
 MTS.OtherMessageDeliveryFields.other-recipient-names are made
 available to the RFC 822 user by use of the "X400-Recipients:" field.
 By taking the recipients at the MTS level, disclosure of recipients
 will be dealt with correctly. However, this conflicts with a desire
 to optimise mail transfer. There is no problem when disclosure of
 recipients is allowed. Similarly, there is no problem if there is
 only one RFC 822 recipient, as the "X400-Recipients" field is only
 given one address.
 There is a problem if there are multiple RFC 822 recipients, and
 disclosure of recipients is prohibited. In this case, discard the
 per-recipient information.
 If any MTS.ORName.directory-name is present, it shall be represented
 in an 822.comment.
 If MTS.OtherMessageDeliveryFields.orignally-intended-recipient-name
 is present, then there has been redirection, or there has been
 distribution list expansion. Distribution list expansion is a per-
 message option, and the information associated with this is
 represented by the "DL-Expansion-History:" field described in Section
 5.3.6. Other information is represented in an 822.comment associated
 with MTS.OtherMessageDeliveryFields.this-recipient-name, The message
 may be delivered to different RFC 822 recipients, and so several
 addresses in the "X400-Recipients:" field may have such comments.
 The non-commented recipient is the RFC 822 recipient. The EBNF of the
 comment is defined by redirect-comment.
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RFC 2156 MIXER January 1998
 redirect-comment = redirect-first *( redirect-subsequent )
 redirect-first = "Originally To:" mailbox "Redirected on"
 date-time "To:" redirection-reason
 redirect-subsequent = mailbox "Redirected Again on"
 date-time "To:" redirection-reason
 redirection-history-item = "intended recipient" mailbox
 "redirected to" redirection-reason
 "on" date-time
 redirection-reason =
 "Recipient Assigned Alternate Recipient"
 / "Originator Requested Alternate Recipient"
 / "Recipient MD Assigned Alternate Recipient"
 / "Directory Look Up"
 / "Alias"
 It is derived from
 MTA.PerRecipientMessageTransferFields.extension.redirection-history.
 The values are taken from the X.400(92) Implementor's guide (Version
 13, July 1995). The first three values are in X.400(88). The
 fourth value is in X.400(92), but has the name "recipient-directory-
 substitution-alternate-recipient". An example of this with two
 redirects is:
 X400-Recipients: postmaster@widget.com (Originally To:
 sales-manager@sales.widget.com
 Redirected on Thu, 30 May 91 14:39:40 +0100
 To: Originator Requested Alternate Recipient
 postmaster@sales.widget.com
 Redirected Again on Thu, 30 May 91 14:41:20 +0100
 To: Recipient MD Assigned Alternate Recipient)
 In addition the following per-recipient services from
 MTS.OtherMessageDeliveryFields.extensions are represented in comments
 if they are used. None of these services can be provided on RFC 822
 networks, and so in general these will be informative strings
 associated with other MTS recipients. In some cases, string values
 are defined. For the remainder, the string value shall be chosen by
 the implementor. If the parameter has a default value, then no
 comment shall be inserted when the parameter has that default value.
 requested-delivery-method
 physical-forwarding-prohibited
 "(Physical Forwarding Prohibited)".
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RFC 2156 MIXER January 1998
 physical-forwarding-address-request
 "(Physical Forwarding Address Requested)".
 physical-delivery-modes
 registered-mail-type
 recipient-number-for-advice
 physical-rendition-attributes
 physical-delivery-report-request
 "(Physical Delivery Report Requested)".
 proof-of-delivery-request
 "(Proof of Delivery Requested)".
4.6.2.3. Delivery Report Generation
 If SMTP is used, the behaviour is specified in Appendix A. In other
 cases, if MTA.PerRecipientMessageTransferFields.per-recipient-
 indicators requires a positive delivery notification, this shall be
 generated by the gateway. Supplementary Information shall be set to
 indicate that the report is gateway generated. This information
 shall include the name of the gateway generating the report.
4.6.3. Message IDs (MTS)
 A mapping from 822.msg-id to MTS.MTSIdentifier is defined. The
 reverse mapping is not needed, as MTS.MTSIdentifier is always mapped
 onto new RFC 822 fields. The value of MTS.MTSIdentifier.local-part
 will facilitate correlation of gateway errors.
 To map from 822.msg-id, apply the standard mapping to 822.msg-id, in
 order to generate an MTS.ORAddress. The Country, ADMD, and PRMD
 components of this are used to generate MTS.MTSIdentifier.global-
 domain-identifier. MTS.MTSIdentifier.local-identifier is set to the
 822.msg-id, including the braces "<" and ">". If this string is
 longer than MTS.ub-local-id-length (32), then it is truncated to this
 length.
 The reverse mapping is not used in this specification. It would be
 applicable where MTS.MTSIdentifier.local-identifier is of syntax
 822.msg-id, and it algorithmically identifies MTS.MTSIdentifier.
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4.7. IPMS Mappings
 All RFC 822 addresses are assumed to use the 822.mailbox syntax.
 This includes all 822.comments associated with the lexical tokens of
 the 822.mailbox. In the IPMS OR Names are encoded as MTS.ORName.
 This is used within the IPMS.ORDescriptor, IPMS.RecipientSpecifier,
 and IPMS.IPMIdentifier. An asymmetrical mapping is defined between
 these components.
4.7.1. RFC 822 -> X.400
 To derive IPMS.ORDescriptor from an RFC 822 address.
 1. Take the address, and extract an EBNF.822-address. Any
 source routing shall be removed. This can be derived trivially
 from either the 822.addr-spec or 822.route-addr syntax. This is
 mapped to MTS.ORName as described above, and used as
 IMPS.ORDescriptor.formal-name.
 2. A string shall be built consisting of (if present):
 - The 822.phrase component if the 822.address is an
 822.phrase 822.route-addr construct.
 - Any 822.comments, in order, retaining the parentheses.
 This string is then encoded into T.61 using a human oriented
 mapping (as described in Section 3.5). If the string is not
 null, it is assigned to IPMS.ORDescriptor.free-form-name.
3. IPMS.ORDescriptor.telephone-number is omitted.
 If IPMS.ORDescriptor is being used in IPMS.RecipientSpecifier,
 IPMS.RecipientSpecifier.reply-request and
 IPMS.RecipientSpecifier.notification-requests are set to default
 values (false and none).
 If the 822.group construct is present, any included 822.mailbox is
 encoded as above to generate a separate IPMS.ORDescriptor. The
 822.group is mapped to T.61 (as described in Section 3.5), and a
 IPMS.ORDescriptor with only an free-form-name component built from
 it.
4.7.2. X.400 -> RFC 822
 Mapping from IPMS.ORDescriptor to RFC 822 address. In the basic
 case, where IPMS.ORDescriptor.formal-name is present, proceed as
 follows.
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 1. Encode IPMS.ORDescriptor.formal-name (MTS.ORName) as
 EBNF.822-address.
 2a. If IPMS.ORDescriptor.free-form-name is present, convert it
 to ASCII or T.61 (Section 3.5), and use this as the 822.phrase
 component of 822.mailbox using the 822.phrase 822.route-addr
 construct.
 2b. If IPMS.ORDescriptor.free-form-name is absent. If
 EBNF.822-address is parsed as 822.addr-spec use this as the
 encoding of 822.mailbox. If EBNF.822-address is parsed as
 822.route 822.addr-spec, then an 822.phrase taken from
 822.local-part is added.
 3 If IPMS.ORDescriptor.telephone-number is present, this is
 placed in an 822.comment, with the string "Tel ". The normal
 international form of number is used. For example:
 (Tel +44-181-333-7777)
 4. If IPMS.ORDescriptor.formal-name.directory-name is present,
 then a text representation is placed in a trailing 822.comment.
 5. If IPMS.RecipientSpecifier.report-request has any non-
 default values, then an 822.comment "(Receipt Notification
 Requested)", and/or "(Non Receipt Notification Requested)",
 and/or "(IPM Return Requested)" may be appended to the address.
 "(Receipt Notification Requested)" may be used to infer "(Non
 Receipt Notification Requested)". The effort of correlating P1
 and P2 information is too great to justify the gateway sending
 Receipt Notifications.
 In RFC 1327, inclusion of these comments was mandatory.
 Experience has shown that the clutter and confusion caused to
 RFC 822 users does not justify the information conveyed.
 Implementors are recommended to not include these comments.
 Unless an application is found where retention of these comments
 is desirable, they will be dropped from the next version.
 6. If IPMS.RecipientSpecifier.reply-request is True, an
 822.comment "(Reply requested)" is appended to the address.
 If IPMS.ORDescriptor.formal-name is absent, IPMS.ORDescriptor.free-
 form-name is converted to ASCII (see section 3.5), and used as
 822.phrase within the RFC 822 822.group syntax. For example:
 Free Form Name ":" ";"
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 Steps 3-6 are then followed.
4.7.3. IP Message IDs
 There is a need to map both ways between 822.msg-id and
 IPMS.IPMIdentifier. This allows for X.400 Receipt Notifications,
 Replies, and Cross References to reference an RFC 822 Message ID,
 which is preferable to a gateway generated ID. A reversible and
 symmetrical mapping is defined. This provides fully reversible
 mappings when messages pass multiple times across the X.400/RFC 822
 boundary.
 An important issue with messages identifiers is mapping to the exact
 form, as many systems use these ids as uninterpreted keys. The use
 of table driven mappings is not always symmetrical, particularly in
 the light of alternative domain names, and alternative management
 domains. For this reason, a purely algorithmic mapping is used. A
 mapping which is simpler than that for addresses can be used for two
 reasons:
 - There is no major requirement to make message IDs "natural"
 - There is no issue about being able to reply to message IDs.
 (For addresses, creating a return path which works is more
 important than being symmetrical).
 The mapping works by defining a way in which message IDs generated on
 one side of the gateway can be represented on the other side in a
 systematic manner. The mapping is defined so that the possibility of
 clashes is low enough to be treated as impossible.
4.7.3.1. 822.msg-id represented in X.400
 IPMS.IPMIdentifier.user is omitted. The IPMS.IPMIdentifier.user-
 relative-identifier is set to a printable string encoding of the
 822.msg-id with the angle braces ("<" and ">") removed. The upper
 bound on this component is 64. The options for handling this are
 discussed in Section 5.1.3.
4.7.3.2. IPMS.IPMIdentifier represented in RFC 822
 The 822.domain of 822.msg-id is set to the value "MHS". The
 822.local-part of 822.msg-id is constructed by building a string of
 syntax EBNF.id-loc from IPMS.IPMIdentifier.
 id-loc ::= [ printablestring ] "*" [ std-or-address ]
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 EBNF.printablestring is the IPMS.IPMIdentifier.user-relative-
 identifier, and EBNF.std-or-address being an encoding of the
 IPMS.IPMIdentifier.user derived according to this specification.
 822.local-part is derived from EBNF.id-loc, if necessary using the
 822.quoted-string encoding. For example:
 <"147*/S=Dietrich/O=Siemens/ADMD=DBP/C=DE/"@MHS>
4.7.3.3. 822.msg-id -> IPMS.IPMIdentifier
 If the 822.local-part can be parsed as:
 [ printablestring ] "*" [ std-or-address ]
 and the 822.domain is "MHS", then this ID was X.400 generated. If
 EBNF.printablestring is present, the value is assigned to
 IPMS.IPMIdentifier.user-relative-identifier. If EBNF.std-or-address
 is present, the OR Address components derived from it are used to set
 IPMS.IPMIdentifier.user.
 Otherwise, this is an RFC 822 generated ID. In this case, set
 IPMS.IPMIdentifier.user-relative-identifier to a printable string
 encoding of the 822.msg-id without the angle braces and omit
 IPMS.IPMID.user.
4.7.3.4. IPMS.IPMIdentifier -> 822.msg-id
 If IPMS.IPMIdentifier.user is absent, and IPMS.IPMIdentifier.user-
 relative-identifier mapped to ASCII and angle braces added parses as
 822.msg-id, then this is an RFC 822 generated ID.
 Otherwise, the ID is X.400 generated. Use the
 IPMS.IPMIdentifier.user to generate an EBNF.std-or-address form
 string. Build the 822.local-part of the 822.msg-id with the syntax:
 [ printablestring ] "*" [ std-or-address ]
 The printablestring is taken from IPMS.IPMIdentifier.user-relative-
 identifier. Use 822.quoted-string if necessary. The 822.msg-id is
 generated with this 822.local-part, and "MHS" as the 822.domain.
4.7.3.5. Phrase form
 In "In-Reply-To:" and "References:", the encoding 822.phrase may be
 used as an alternative to 822.msg-id. To map from 822.phrase to
 IPMS.IPMIdentifier, assign IPMS.IPMIdentifier.user-relative-
 identifier to the phrase. When mapping from IPMS.IPMIdentifier for
 "In-Reply-To:" and "References:", if IPMS.IPMIdentifier.user is
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RFC 2156 MIXER January 1998
 absent and IPMS.IPMIdentifier.user-relative-identifier does not parse
 as 822.msg-id, generate an 822.phrase rather than adding the domain
 MHS.
4.7.3.6. RFC 987 backwards compatibility
 The mapping defined here is different to that used in RFC 987, as the
 RFC 987 mapping lead to changed message IDs in many cases. Fixing
 the problems is preferable to retaining backwards compatibility. An
 implementation of this standard may recognise message IDs generated
 by RFC 987. This is not recommended.
 RFC 987 generated encodings may be recognised as follows. When
 mapping from X.400 to RFC 822, if the IPMS.IPMIdentifier.user-
 relative-identifier is "RFC-822" the id is RFC 987 generated. When
 mapping from RFC 822 to X.400, if the 822.domain is not "MHS", and
 the 822.local-part can be parsed as
 [ printablestring ] "*" [ std-or-address ]
 then it is RFC 987 generated. In each of these cases, it is
 recommended to follow the RFC 987 rules.
Chapter 5 - Detailed Mappings
 This chapter specifies detailed mappings for the functions outlined
 in Chapters 1 and 2. It makes extensive use of the notations and
 mappings defined in Chapters 3 and 4.
5.1. RFC 822 -> X.400: Detailed Mappings
 The mapping of RFC 822/MIME messages to X.400 InterPersonal Messages
 is described in Sections 5.1.1 to 5.1.7. Mapping of NOTARY format
 delivery status notifications, which are all messages of type
 multipart/report and subtype delivery-status-notifications to X.400
 delivery reports is covered in Section 5.1.8.
5.1.1. Basic Approach
 A single IP Message is generated from an RFC 822 message. The RFC
 822 headers are used to generate the IPMS.Heading.
 Some RFC 822 fields cannot be mapped onto a standard IPM Heading
 field, and so an extended field is defined in Section 5.1.2. This is
 then used for fields which cannot be mapped onto existing services.
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 The message is submitted to the MTS, and the services required can be
 defined by specifying MTS.MessageSubmissionEnvelope. A few
 parameters of the MTA Abstract service are also specified, which are
 not in principle available to the MTS User. Use of these services
 allows RFC 822 MTA level parameters to be carried in the analogous
 X.400 service elements. The advantages of this mapping far outweigh
 the layering violation.
5.1.2. X.400 Extension Field
 An IPMS Extension is defined:
 rfc-822-field HEADING-EXTENSION
 VALUE RFC822FieldList
 ::= id-rfc-822-field-list
 RFC822FieldList ::= SEQUENCE OF RFC822Field
 RFC822Field ::= IA5String
 The Object Identifier id-rfc-822-field-list is defined in Appendix D.
 To encode any RFC 822 Header using this extension, an RFC822Field
 element is built using the 822.field omitting the trailing CRLF
 (e.g., "Fruit-Of-The-Day: Kiwi Fruit"). All fields shall be unfolded.
 There shall be no space before the ":". The reverse mapping builds
 the RFC 822 field in a straightforward manner. This RFC822Field is
 appended to the RFC822FieldList, which is added to the IPM Heading as
 an extension field.
5.1.3. Generating the IPM
 The IPM (IPMS Service Request) is generated according to the rules of
 this section. The IPMS.IPM.body is generated from the RFC 822 message
 body in the manner described in Section 5.1.5.
 If no specific 1988 features are used, the IPM generated is encoded
 as content type 2. Otherwise, it is encoded as content type 22. The
 latter will always be the case if extension heading fields are
 generated.
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RFC 2156 MIXER January 1998
 When generating the IPM, the issue of upper bounds are handled as
 follows. Truncate fields to the upper bounds specified in X.400.
 This will prevent problems with UAs which enforce upper bounds, but
 will sometimes discard useful information. This approach will cause
 more problems for some fields than others (e.g., truncating an OR
 Address component that would be used to route a reply would be a more
 severe problem than truncating a Free Form Name). If the Free Form
 name is truncated, it shall be done so that it does not break RFC 822
 comments and RFC 1522 encoding.
 Note:This approach removes a choice of options given in RFC 1327,
 based on operational experience.
 The rest of this section concerns IPMS.IPM.heading (IPMS.Heading).
 The only mandatory component of IPMS.Heading is the
 IPMS.Heading.this-IPM (IPMS.IPMIdentifier). A default is generated
 by the gateway. With the exception of "Received:", the values of
 multiple fields are merged (e.g., If there are two "To:" fields, then
 the mailboxes of both are merged to generate a single list which is
 used in the IPMS.Heading.primary-recipients. Information shall be
 generated from the standard RFC 822 Headers as follows:
 Date:
 Ignore (Handled at MTS level)
 Received:
 Ignore (Handled at MTA level)
 Message-Id:
 Mapped to IPMS.Heading.this-IPM. For these, and all other
 fields containing 822.msg-id the mappings of Chapter 4 are used
 for each 822.msg-id.
 From:
 If Sender: is present, this is mapped to
 IPMS.Heading.authorizing-users. If not, it is mapped to
 IPMS.Heading.originator. For this, and other components
 containing addresses, the mappings of Chapter 4 are used for
 each address.
 Sender:
 Mapped to IPMS.Heading.originator. Because X.400 does not have
 the same From/Sender distinction as RFC 822, this mapping is not
 always natural and may lead to unexpected results in some cases.
 Reply-To:
 Mapped to IPMS.Heading.reply-recipients.
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RFC 2156 MIXER January 1998
 To: Mapped to IPMS.Heading.primary-recipients
 Cc: Mapped to IPMS.Heading.copy-recipients.
 Bcc: Mapped to IPMS.Heading.blind-copy-recipients if there is at
 least one BCC: recipient. If there are no recipients in this
 field, it shall either be mapped to a zero length sequence or
 mapped to a single recipient that has a free from name "BCC" and
 no other addressing information. This alternate treatment is
 allowed because some X.400 systems cannot handle a zero lenght
 sequence of addresses.
 In-Reply-To:
 If there is one value, it is mapped to IPMS.Heading.replied-to-
 IPM, using the 822.phrase or 822.msg-id mapping as appropriate.
 If there are multiple values, this cannot be done as the X.400
 heading is single valued. In this case no IPMS.Heading.replied-
 to-IPM is generated and the values are mapped to
 IPMS.Heading.related-IPMs, along with any values from a
 "References:" field.
 References:
 Mapped to IPMS.Heading.related-IPMs.
 Keywords:
 Mapped onto a heading extension.
 Subject:
 Mapped to IPMS.Heading.subject. The field-body uses the human
 oriented mapping referenced in Section 3.3.4.
 Comments:
 Mapped onto a heading extension.
 This is a change from 1327, which specified to generate an
 IPMS.BodyPart of type IPMS.IA5TextBodyPart with
 IPMS.IA5TextBodyPart.parameters.repertoire set to the default
 (ia5), containing the value of the fields, preceded by the
 string "Comments: " and that this body part shall precede the
 other one. Experience has shown that this complexity is not
 justified. This text is retained to facilitate backwards
 compatibility.
 Encrypted:
 Mapped onto a heading extension.
 Resent-*
 Mapped onto a heading extension.
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 Note that it would be possible to use a ForwardedIPMessage for
 these fields, but the semantics are (arguably) slightly
 different, and it is probably not worth the effort.
 Content-Language:
 This field is defined in RFC 1766 [7]. Map the first two
 characters of each value given onto the IPM Languages extension.
 If any comments or values longer than two characters occur, a
 header extension shall also be generated.
 Other Fields
 In particular X-* fields, and "illegal" fields in common usage
 (e.g., "Fruit-of-the-day:") are mapped onto a heading extension,
 unless covered by another section or appendix of this
 specification. The same treatment is applied to RFC 822 fields
 where the content of the field does not conform to RFC 822
 (e.g., a Date: field with unparseable syntax).
 The mapping of the following headings is defined in RFC 2157.
 MIME-Version: 5
 Content-Transfer-Encoding:
 Content-Type
 Content-ID
 Content-Description
5.1.4. Generating the IPM Body
 Generation of the IPM Body is defined in RFC 2157.
5.1.5. Mappings to the MTS Abstract Service
 The MTS.MessageSubmissionEnvelope comprises
 MTS.PerMessageSubmissionFields, and
 MTS.PerRecipientMessageSubmissionFields. The mandatory parameters
 are defaulted as follows.
 MTS.PerMessageSubmissionFields.originator-name
 This is always generated from SMTP, as defined in Chapter 4.
 MTS.PerMessageSubmissionFields.content-type
 Set to the value implied by the encoding of the IPM (2 or 22).
 MTS.PerRecipientMessageSubmissionFields.recipient-name
 These will always be supplied from SMTP, as defined in Chapter 4.
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 Optional components are omitted, and default components defaulted.
 This means that disclosure of recipients is prohibited and conversion
 is allowed. There are two exceptions to the defaulting. For
 MTS.PerMessageSubmissionFields.per-message-indicators, the following
 settings are made:
 - Alternate recipient is allowed, as it seems desirable to
 maximise the opportunity for (reliable) delivery.
 If SMTP is used, Appendix A shall be followed in setting these
 parameters.
 The trace is set to indicate conversion (described below) and the
 encoded information types in the trace is derived from the message
 generated by the gateway, and shall reflect all body parts (including
 those in enclosed messages). In addition it shall include the
 Encoded Information Type "eit-mixer", which is defined in Appendix D.
 The presence of the EIT will indicate to the X.400 recipient that a
 MIXER conversion has occurred.
 MTS.PerMessageSubmissionFields.original-encoded-information-types
 will include all of the values used in the trace, unless specified
 otherwise in RFC 2157.
 This type of conversion will prevent the normal loop detection from
 working in certain circumstances, and introduces the possiblity of
 gateway loops. MIXER gateways shall therefore count the number of
 MIXER conversions made. If this count exceeds five in one direction,
 the message shall be treated as if a loop has been detected.
 The MTS.PerMessageSubmissionFields.content-correlator is encoded as
 IA5String, and contains the Subject:, Message-ID:, Date:, and To:
 fields (if present) in this order. This includes the strings
 "Subject:", "Date:", "To:", "Message-ID:", and appropriate folding to
 make the field appear readable. This shall be truncated to MTS.ub-
 content-correlator-length (512) characters. In addition, if there is
 a "Subject:" field, the MTS.PerMessageSubmissionFields.content-
 identifier, is set to a printable string representation of the
 contents of it. If the length of this string is greater than
 MTS.ub-content-id-length (16), it shall be truncated to 13 characters
 and the string "..." appended. Both are used, due to the much larger
 upper bound of the content correlator, and that the content id is
 available in X.400(1984).
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5.1.6. Mappings to the MTA Abstract Service
 There is a need to map directly onto some aspects of the MTA Abstract
 service, for the following reasons:
 - So the MTS Message Identifier can be generated from the RFC
 822 Message-ID:.
 - So that the submission date can be generated from the
 822.Date.
 - To prevent loss of trace information
 - To prevent RFC 822/X.400 looping caused by distribution
 lists or redirects
 The following mappings are defined.
 Message-Id:
 If this is present and no Resent: fields are present, the
 MTA.PerMessageTransferFields.message-identifier may be generated
 from it, using the mappings described in Chapter 4.
 This mapping arguably generates messages which do not conform to
 US GOSIP (1984 version only), which states:
 6.7.e MPDU Identifier Validation
 (1) Validation of the GlobalDomainIdentifier component of the MPDU
 Identifier is performed on reception of a message (i.e. the result
 of a TRANSFER.Indication).
 (2) The country name should be known to the validating domain, and
 depending on the country name, validation of the
 ADMD name may also be possible.
 (3) Additional validation of the GlobalDomainIdentifier is
 performed against the corresponding first entry in the
 TraceInformation. If inconsistencies are found during the
 comparison, a non-delivery notice with the above defined reason
 and diagnostic code is generated.
 (4) A request will be generated to the CCITT for a more meaningful
 diagnostic code (such as "InconsistentMPUTIdentifier").
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 This applies to ADMDs only, and is specified in the 1984 version and
 not the 1988 version. Conformance depends on the interpretation of
 "inconsistency". The specification makes the most sensible choice,
 and so is not being changed in the update from RFC 1327.
 Date: (and Resent-Date:)
 If one or more Resent-Date: fields is present, the most recent
 Resent-Date: field shall be used instead of the Date: field in the
 following description.
 The Date: field is used to set the first component of
 MTA.PerMessageTransferFields.trace-information
 (MTA.TraceInformationElement). The SMTP originator is mapped into
 an MTS.ORAddress, and used to derive
 MTA.TraceInformationElement.global-domain-identifier. The
 optional components of MTA.TraceInformationElement.domain-
 supplied-information are omitted, and the mandatory components are
 set as follows:
 MTA.DomainSuppliedInformation.arrival-time
 This is set to the date derived from Date:
 MTA.DomainSuppliedInformation.routing-action
 Set to relayed.
 The first element of MTA.PerMessageTransferFields.internal-trace-
 information is generated in an analogous manner, although this can
 be dropped later in certain circumstances (see the procedures for
 "Received:"). The MTA.InternalTraceInformationElement.mta-name is
 derived from the 822.domain in the 822 MTS Originator address.
 Received:
 All RFC 822 trace is used to derive
 MTA.PerMessageTransferFields.trace-information and
 MTA.PerMessageTransferFields.internal-trace-information.
 Processing of Received: lines follows processing of Date:, and is
 done from the bottom to the top of the RFC 822 header (i.e., in
 chronological order). When other trace elements (in particular
 X400-Received:) are processed the relative ordering (top to
 bottom of the header) shall be retained correctly.
 The initial element of MTA.PerMessageTransferFields.trace-
 information shall be generated from Date: as described above,
 unless the message has previously been in X.400, when it will be
 derived from the X.400 trace information.
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 For each Received: field, the following processing shall be done.
 If the "by" part of the received is present and there is an
 available MCGAM which can map this domain, use it to derive an
 MTS.GlobalDomainIdentifier. Otherwise MTS.GlobalDomainIdentifier
 is set from local information. If this is different from the one
 in the last element of MTA.PerMessageTransferFields.trace-
 information (MTA.TraceInformationElement.global-domain-identifier)
 create a new MTA.TraceInformationElement, and optionally remove
 MTA.PerMessageTransferFields.internal-trace-information.
 Requirements on trace stripping are discussed below.
 Then add a new element (MTA.InternalTraceInformationElement) to
 MTA.PerMessageTransferFields.internal-trace-information, creating
 this if needed. This shall be done, even if nter-MD trace is
 created. The MTA.InternalTraceInformationElement.global-domain-
 identifier is set to the value derived. The
 MTA.InternalTraceInformationElement.mta-supplied-information
 (MTA.MTASuppliedInformation) is set as follows:
 MTA.MTASuppliedInformation.arrival-time
 Derived from the date of the Received: line
 MTA.MTASuppliedInformation.routing-action
 Set to relayed
 The MTA.InternalTraceInformationElement.mta-name is taken from the
 "by" component of the "Received:" field, truncated to MTS.ub-mta-
 name-length (32). For example:
 Received: from computer-science.nottingham.ac.uk by
 vs6.Cs.Ucl.AC.UK via Janet with NIFTP id aa03794;
 28 Mar 89 16:38 GMT
 Generates the string
 vs6.Cs.Ucl.AC.UK
 The gateway shall add in a single element of trace information,
 reflecting the gateway's local information and the time of
 conversion. The MTA.InternalTraceInformationElement.mta-supplied-
 information (MTA.MTASuppliedInformation) is set as follows:
 MTA.DomainSuppliedInformation.arrival-time
 Set to the time of conversion
 MTA.DomainSuppliedInformation.routing-action
 Set to relayed
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RFC 2156 MIXER January 1998
 MTA.AdditionalAcctions.converted-encoded-information-types Set to
 correct set of EITs for the message that is generated by the gateway.
 This trace element will thus reflect gateway operation as a
 conversion.
 This trace generation will often lead to generation of substantial
 amounts of trace information, which does not reflect X.400 transfers.
 Stripping of some of this trace may be necessary in some operational
 environments. This stripping shall be considered a function of the
 associated X.400 MTA, and not of the MIXER gateway.
5.1.7. Mapping New Fields
 This specification defines a number of new fields for Reports,
 Notifications and IP Messages. A gateway conforming to this
 specification shall map all of these fields to X.400, except as
 defined below.
 The mapping of two extended fields is particularly important, in
 order to prevent looping. "DL-Expansion-History:" is mapped to
 MTA.PerMessageTransferFields.extensions.dl-expansion-history X400-
 Received: shall be mapped to MTA.PerMessageTransferFields.trace-
 information and MTA.PerMessageTransferFields.internal-trace-
 information. In cases where X400-Received: is present, the usual
 mapping of Date: to generate the first element of trace shall not be
 done. This is because the message has come from X.400, and so the
 first element of trace can be taken from the first X400-Received:.
 The following fields shall not be mapped, and shall be
 - Discarded-X400-MTS-Extensions:
 - Message-Type:
 - Discarded-X400-IPMS-Extensions:
 - X400-Content-Type:
 - X400-Originator:
 - X400-Recipients:
 - X400-MTS-Identifier: Mapping this field would be useful in
 some circumstances, but very dangerous in others (e.g.,
 following an internet list expansion). Therefore it is not
 mapped.
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5.1.8. Mapping Delivery Status Notifications to X.400
5.1.8.1. Basic Model
 Internet Mail delivery status notifications (DSN) are mapped to X.400
 delivery reports. With message mapping, information without a
 mapping is carried by an IPM Extension. This cannot be done for
 delivery reports. Two mechanisms are used for information where
 there is not a direct mapping.
 The first mechanism is to define extensions, which allow all of the
 DSN information to be carried in the delivery report. This is not
 completely satisfactory for two reasons:
 1. User defined extensions are supported by the ISO version of
 the standard, but not the CCITT one. Therefore,
 implementation support for these extensions will not be
 universal.
 2. X.400 User Agent implementations will not in general
 recognise these extensions. Therefore, although the
 information will be present, it will often not be available
 to the user. This may be very problematic, as this
 information may be critical to diagnosing the reason for a
 failure.
 Therefore a second mechanism is defined. This shall always be used
 when the DSN contains non-delivery information, and may be used in
 other cases. This mechanism is to map the whole DSN (as if it were
 an ordinary multipart) into the return of content. This will make
 the DSN information available as a text body part in the outer
 message, with the real returned content as an enclosed message. This
 mechanism will ensure that information is not lost at the gateway.
5.1.8.2. DSN Extensions
 Two X.400 MTS extensions are defined as follows:
 dsn-header-list EXTENSION
 RFC822FieldList
 ::= id-dsn-header-list
 dsn-field-list EXTENSION
 RFC822FieldList
 ::= id-dsn-field-list
Kille Standards Track [Page 81]
RFC 2156 MIXER January 1998
 The Object Identifiers id-dsn-header-list and id-dsn-field-list are
 defined in Appendix D. Theses extensions are used in the same way as
 the IPM extension rfc-822-field, described in Section 5.1.2. These
 extensions may only be used with ISO-10021, and not X.400 (which does
 not allow user extensions at the MTS level).
5.1.8.3. DSN to Delivery Report Mapping
 Some DSNs are mapped to Delivery Reports and some to IPMs, according
 to the value of the action field. The mapping to an IPM is exactly
 as for a normal IPM mapping. The choice of IPM and Delivery report
 is made for each reported recipient. If this choice is different
 for different reported recipients both a Delivery Report and an IPM
 shall be generated.
 Reports are not be submitted in the X.400 model, and so the report
 submission is considered in terms of the MTA Abstract Service. An
 MTA.Report is constructed. The MTA.ReportTransferEnvelope.report-
 identifier is generated from the Message-Id of the DSN (if present)
 and otherwise generated as the MTA would generate one for a submitted
 message.
 The DSN has an RFC 822 header. Trace is mapped in the same manner as
 for a message to MTA.ReportTransferEnvelope.trace-information. All
 other headers are used to create a dsn-header-list extension, which
 is added to MTA.PerReportTransferFields.extensions. The DSN will
 have a single SMTP recipient. This is mapped to the
 MTA.ReportTransferEnvelope.report-destination-name.
 The DSN is then treated as a normal MIME message, and an X.400 IPM is
 generated. This IPM is used as
 MTA.PerReportTransferFields.returned-content, and its type is used to
 set MTA.PerReportTransferFields.content-type. The DSN body part is
 mapped as if it was IA5 text/plain.
 The mandatory MTA.PerReportTransferFields.subject-identifier shall be
 generated from the DSN.per-message-field original-envelope-id, if
 this starts with the string "X400-MTS-Identifier: ", and derived from
 the rest of the field, which is encoded as EBNF.mts-msg-id. In other
 cases, this field shall be generated by the MIXER Gateway.
 All other mappings are made from the DSN body part. A dsn-field-list
 extension is created and added to
 MTA.ReportTransferFields.extensions. This is referred to as the per
 report extension list. The DSN.per-message-fields are mapped as
 follows:
Kille Standards Track [Page 82]
RFC 2156 MIXER January 1998
 original-envelope-id-field
 reporting-mta-field
 dsn-gateway-field
 received-from-mta-field
 arrival-date-field
 extension-field
 other
 All of these fields are added to the per report extension list.
 Currently there are no other mappings defined.
 Each reported recipient is considered in turn, and a
 MTA.PerRecipientReportTransferFields created for each. The
 parameters of this are defaulted as follows:
 originally-specified-recipient-number
 In general, these are not available, and so are assigned
 incrementally.
 last-trace-information
 The arrival-time is generated from DSN.arrival-date if present,
 and if not from the Date: of the DSN. This is a strucutred field,
 and the Report element contains the key information on the
 recipient. For a DeliveryReport, the type-ofMTS-user is defaulted
 to public and the message-deliery-time is set to the same as the
 arrival-time. For a NonDeliveryReport, the code mappings are
 define in Section 5.1.8.4.
 A dsn-field-list extension is created and added to
 MTA.PerRecipientTransferFields.extensions. This is referred to as
 the per recipient extension list. The DSN.per-recipient-fields are
 mapped as follows
 original-recipient-field
 Mapped to MTA.PerRecipientReportTransferFields.originally-
 intended-recipient-name.
 final-recipient-field
 Mapped to MTA.PerRecipientReportTransferFields.actual-recipient-
 name.
 action-field
 If this is set to "failed", a non-delivery report is generated.
 If this is set to "delivered" a delivery report is generated.
 Bit one or two of MTA.PerRecipientTransferFields.per-recipient-
 indicators is set accordingly. This also controls the encoding of
 MTA.PerRecipientTransferFields.last-trace-information, and the
 selection of the report type.
Kille Standards Track [Page 83]
RFC 2156 MIXER January 1998
 For other values of the action-field ("delayed", "relayed",
 "expanded"), an IPM is generated. This enables the status
 information to be communicated to the X.400 user, without the
 confusion of multiple delivery reports.
 status-field
 This is added to the per report extension list. For non-delivery,
 it is also used to generate the reason and diagnostic codes
 contained within MTA.PerRecipientReportTransferFields.last-trace.
 The mappings are defined below.
 remote-mta-field
 diagnostic-code-field
 last-attempt-date-field
 will-retry-until-field
 extension-field
 other
 All of these fields are added to the per recipient extension list.
5.1.8.4. Status Value Mappings
 Status values are mapped to X.400 reason and diagnostic codes as
 follows.
 If a status value is found that is not in this table, the gateway may
 use the same mapping as for "X.n.0" (1/None or 0/None), or it may map
 to another, configurable code. Implementors are requested to forward
 new codes to the mixer list for inclusion in future versions of this
 standard. So for instance. "5.2.37", currently undefined, would map
 onto the same as "5.2.0", namely 1/None.
Kille Standards Track [Page 84]
RFC 2156 MIXER January 1998
DSN code Meaning X400 code Meaning
X.0.0 Other status 1/None
X.1.0 Other Address Status 1/None
X.1.1 Bad mailbox address 1/0 Unrecognized
X.1.2 Bad system address 1/0 Unrecognized
X.1.3 Bad mailbox address syntax 1/0 Unrecognized
X.1.4 Mailbox address ambiguous 1/1
X.1.5 Only used for positive reports, not applicable
X.1.6 Destination mailbox has moved 1/43 New addr unknown
X.1.7 Bad sender's mailbox address syntax 1/11 Invalid arguments
X.1.8 Bad sender's system address 1/11 Invalid arguments
X.2.0 Other or undefined mailbox status 1/None
X.2.1 Mailbox disabled, not accepting 1/4 Recipient unavail
X.2.2 Mailbox full 1/4
X.2.3 Message length exceeds admin limit. 1/7 Content too long
X.2.4 Mailing list expansion problem 1/30 DL expansion fail
X.3.0 Other or undefined system status 0/None
X.3.1 System full 1/2 MTS congestion
X.3.2 System not accepting network messages 1/2 MTS congestion
X.3.3 System not capable of selected feat 1/18 Unsupp crit func
X.3.4 Message too big for system 1/7
X.3.5 System incorrectly configured 1/None
X.4.0 Other or undefined network or routing 0/None
X.4.1 No answer from host 0/None
X.4.2 Bad connection 0/None
X.4.3 Routing server failure 6/None Dir op unsucc.
X.4.4. Unable to route 0/None
X.4.5 Network congestion 1/2 MTS congest.
X.4.6 Routing loop detected 1/3
X.4.7 Delivery time expired 1/5
X.5.0 Other or undefined protocol status 1/None
X.5.1 Invalid command 1/14 Protocol viol.
X.5.2 Syntax error 1/14
X.5.3 Too many recipients 1/16
X.5.4 Invalid command arguments 1/14
X.5.5 Wrong protocol version 1/18 Unsupp.crit.func
Kille Standards Track [Page 85]
RFC 2156 MIXER January 1998
X.6.0 Other or undefined media error 2/None Conv. not perf
X.6.1 Media not supported 1/6 EIT unsupp.
X.6.2 Conversion required and prohibited 1/9
X.6.3 Conversion required but not supported 2/8
X.6.4 Conversion with loss performed POSITIVE only
X.6.5 Conversion failed 2/47 Unable to downgrade
X.7.0 Other or undefined security status 1/46
X.7.1 Delivery not authorized, message ref 1/29 No DL submit perm
X.7.2 Mailing list expansion prohibited 1/28
X.7.3 Security conversion req but not poss 1/46 Secure mess. error
X.7.4 Security features not supported 1/46
X.7.5 Cryptographic failure 1/46
X.7.6 Cryptographic algorithm not supported 1/46
X.7.7 Message integrity failure 1/46
5.1.8.5. DSNs that originated in X.400
 The mapping of X.400 delivery reports to DSNs will in general provide
 sufficient information to make a useful reverse mapping. Messages
 will often be mapped multiple times, commonly due to forwarding
 messages and to distribution lists. Multiple mappings for delivery
 reports will be a good deal less common. For this reason, the
 reverse mapping of the X.400 DSN extensions defined in MIXER is
 optional.
5.2. Return of Contents
 RFC 1327 offered two approaches for return of content, as this
 service is optional in X.400 and expected in RFC 822. MIXER simply
 requires that a gateway requests the return of content service from
 X.400.
5.3. X.400 -> RFC 822: Detailed Mappings
5.3.1. Basic Approach
 A single RFC 822 message is generated from the incoming IP Message,
 Report, or IP Notification. All IPMS.BodyParts are mapped onto a
 single RFC 822 body. Other services are mapped onto RFC 822 header
 fields. Where there is no appropriate existing field, new fields are
 defined for IPMS, MTS and MTA services.
 The gateway mechanisms will correspond to MTS Delivery. As with
 submission, there are aspects where the MTA (transfer) services are
 also used. In particular, there is an optimisation to allow for
 multiple SMTP recipients.
Kille Standards Track [Page 86]
RFC 2156 MIXER January 1998
5.3.2. RFC 822 Settings
 An RFC 822 Message has a number of mandatory fields in the RFC 822
 Header. Some SMTP services mandate specification of an SMTP
 Originator. Even in cases where this is optional, it is usually
 desirable to specify a value. The following defaults are defined,
 which shall be used if the mappings specified do not derive a value:
 SMTP Originator
 If this is not generated by the mapping (e.g., for a Delivery
 Report), a value pointing at a gateway administrator shall be
 assigned.
 Date:
 A value will always be generated
 From:
 If this is not generated by the mapping, it is assigned equal to
 the SMTP Originator. If this is gateway generated, an appropriate
 822.phrase shall be added.
 At least one recipient field
 If no recipient fields are generated, a field "To: list:;", shall
 be added.
 This will ensure minimal RFC 822 compliance. When generating RFC 822
 headers, folding may be used. It is recommended to do this,
 following the guidelines of RFC 822.
5.3.3. Basic Mappings
5.3.3.1. Encoded Information Types
 This mapping from MTS.EncodedInformationTypes is needed in several
 disconnected places. EBNF is defined as follows:
 encoded-info = 1#encoded-type
 encoded-type = built-in-eit / object-identifier
Kille Standards Track [Page 87]
RFC 2156 MIXER January 1998
 built-in-eit = "Undefined" ; undefined (0)
 / "Telex" ; tLX (1)
 / "IA5-Text" ; iA5Text (2)
 / "G3-Fax" ; g3Fax (3)
 / "TIF0" ; tIF0 (4)
 / "Teletex" ; tTX (5)
 / "Videotex" ; videotex (6)
 / "Voice" ; voice (7)
 / "SFD" ; sFD (8)
 / "TIF1" ; tIF1 (9)
 MTS.EncodedInformationTypes is mapped onto EBNF.encoded-info.
 MTS.EncodedInformationTypes.non-basic-parameters is ignored. Built
 in types are mapped onto fixed strings (compatible with X.400(1984)
 and RFC 987), and other types are mapped onto EBNF.object-identifier.
5.3.3.2. Global Domain Identifier
 The following simple EBNF is used to represent
 MTS.GlobalDomainIdentifier:
 global-id = std-or-address
 This is encoded using the std-or-address syntax, for the attributes
 within the Global Domain Identifier.
5.3.4. Mappings from the IP Message
 Consider that an IPM has to be mapped to RFC 822. The IPMS.IPM
 comprises an IPMS.IPM.heading and IPMS.IPM.body. The heading is
 considered first. Some EBNF for new fields is defined:
ipms-field = "Supersedes" ":" 1*msg-id
 / "Expires" ":" date-time
 / "Reply-By" ":" date-time
 / "Importance" ":" importance
 / "Sensitivity" ":" sensitivity
 / "Autoforwarded" ":" boolean
 / "Incomplete-Copy" ":"
 / "Content-Language" ":" 1#language
 / "Message-Type" ":" message-type
 / "Discarded-X400-IPMS-Extensions" ":" 1#object-identifier
 / "Autosubmitted" ":" autosubmitted
importance = "low" / "normal" / "high"
Kille Standards Track [Page 88]
RFC 2156 MIXER January 1998
sensitivity = "Personal" / "Private" /
 "Company-Confidential"
language = 2*ALPHA [ "(" language-description ")" ]
 language-description = printable-string
message-type = "Delivery Report"
 / "InterPersonal Notification"
 / "Multiple Part"
autosubmitted = "not-auto-submitted"
 / "auto-generated"
 / "auto-replied"
 / "auto-forwarded"
 The mappings and actions for the IPMS.Heading are now specified for
 each element. Addresses and Message Identifiers are mapped according
 to Chapter 4. Other mappings are explained, or are straightforward
 (algorithmic). If a field with addresses contains zero elements, it
 shall be discarded, except for IPMS.Heading.blind-copy-recipients,
 which can be mapped onto BCC: (the only RFC 822 field which allows
 zero recipients).
 IPMS.Heading.this-IPM
 Mapped to "Message-ID:".
 IPMS.Heading.originator
 If IPMS.Heading.authorizing-users is present this is mapped to
 Sender:, if not to "From:".
 IPMS.Heading.authorizing-users
 Mapped to "From:".
 IPMS.Heading.primary-recipients
 Mapped to "To:".
 IPMS.Heading.copy-recipients
 Mapped to "Cc:".
 IPMS.Heading.blind-copy-recipients
 Mapped to "Bcc:".
 IPMS.Heading.replied-to-ipm
 Mapped to "In-Reply-To:".
Kille Standards Track [Page 89]
RFC 2156 MIXER January 1998
 IPMS.Heading.obsoleted-IPMs
 Mapped to the extended RFC 822 field "Supersedes:". The replaces
 the RFC 1327 field "Obsoletes:". Reverse mapping of the RFC 1327
 field may be supported.
 IPMS.Heading.related-IPMs
 Mapped to "References:".
 IPMS.Heading.subject
 Mapped to "Subject:". The contents are converted to ASCII or T.61
 (as defined in Section 3.5). CRLF will not be present in a valid
 X.400 field. Any CRLF present are not mapped, but are used as
 points at which the subject field shall be folded, unless an RFC
 1522 encoding is used.
 IPMS.Heading.expiry-time
 Mapped to the extended RFC 822 field "Expires:". The replaces the
 RFC 1327 field "Expiry-Date:". Reverse mapping of the RFC 1327
 field may be supported.
 IPMS.Heading.reply-time
 Mapped to the extended RFC 822 field "Reply-By:".
 IPMS.Heading.reply-recipients
 Mapped to "Reply-To:".
 IPMS.Heading.importance
 Mapped to the extended RFC 822 field "Importance:".
 IPMS.Heading.sensitivity
 Mapped to the extended RFC 822 field "Sensitivity:".
 IPMS.Heading.autoforwarded
 Mapped to the extended RFC 822 field "Autoforwarded:".
 The standard extensions (Annex H of X.420 / ISO 10021-7) are mapped
 as follows:
 incomplete-copy
 Mapped to the extended RFC 822 field "Incomplete-Copy:".
 language
 Mapped to the RFC 822 field "Content-Language:", defined in RFC
 1766 [7]. This mapping may be made without loss of information.
 auto-submitted
 Map to the extended RFC 822 field "Autosubmitted:".
Kille Standards Track [Page 90]
RFC 2156 MIXER January 1998
 If the RFC 822 extended header is found, this shall be mapped onto an
 RFC 822 header, as described in Section 5.1.2.
 If a non-standard extension is found, it shall be discarded, unless
 the gateway understands the extension and can perform an appropriate
 mapping onto an RFC 822 header field. If extensions are discarded,
 the list is indicated in the extended RFC 822 field "Discarded-X400-
 IPMS-Extensions:".
5.3.4.1. Mapping the IPMS Body
 The mapping of the IPMS Body is defined in RFC 2157.
5.3.4.2. Example Message
 An example message, illustrating a number of aspects is given below.
Received: from mhs-relay.ac.uk by bells.cs.ucl.ac.uk via JANET with
 NIFTP id <7906-0@bells.cs.ucl.ac.uk>;
 1991年5月30日 18:24:55 +0100
X400-Received: by mta "mhs-relay.ac.uk" in /PRMD=uk.ac/ADMD= /C=gb/;
 Relayed; 1991年5月30日 18:23:26 +0100
X400-Received: by /PRMD=HMG/ADMD=GOLD 400/C=GB/; Relayed;
 1991年5月30日 18:20:27 +0100
Message-Type: Multiple Part
Date: 1991年5月30日 18:20:27 +0100
X400-Originator: Stephen.Harrison@gosip-uk.hmg.gold-400.gb
X400-MTS-Identifier:
 [/PRMD=HMG/ADMD=GOLD 400/C=GB/;PC1000-910530172027-57D8]
Original-Encoded-Information-Types: ia5
X400-Content-Type: P2-1984 (2)
X400-Content-Identifier: Email Problems
From: Stephen.Harrison@gosip-uk.hmg.gold-400.gb (Tel +44 71 217 3487)
Message-ID: <PC1000-910530172027-57D8*@MHS>
To: Jim Craigie <NTIN36@gec-b.rutherford.ac.uk>,
 Tony Bates <tony@ean-relay.ac.uk>,
 Steve Kille <S.Kille@cs.ucl.ac.uk>
Subject: Email Problems
Sender: Stephen.Harrison@gosip-uk.hmg.gold-400.gb
MIME-Version: 1.0
Content-Type: multipart/mixed; boundary=boundary-1
--boundary-1
Content-Type: text/plain; charset=US-ASCII
Hope you gentlemen.......
Kille Standards Track [Page 91]
RFC 2156 MIXER January 1998
Regards,
Stephen Harrison
UK GOSIP Project
--boundary-1
Content-Type: message/rfc822
From: Urs Eppenberger <Eppenberger@verw.switch.ch>
Message-ID:
<562*/S=Eppenberger/OU=verw/O=switch/PRMD=SWITCH/ADMD=ARCOM/C=CH/@MHS>
To: "Stephen.Harrison" <Stephen.Harrison@gosip-uk.hmg.gold-400.gb>
Cc: kimura@bsdarc.bsd.fc.nec.co.jp
Subject: Response to Email link
Content-Type: multipart/mixed; boundary=boundary-2
--boundary-2
Dear Mr Harrison......
--boundary-2--
--boundary-1--
5.3.5. Mappings from an IP Notification
 Because of the service setting, IP Notifications will not usually
 need to be mapped by a MIXER gateway. A message is generated, with
 the following fields:
 From:
 Set to the IPMS.IPN.ipn-originator.
 To: Set to the recipient from MTS.MessageSubmissionEnvelope.
 If there have been redirects, the original address shall be used.
 Subject:
 Set to the string "X.400 Inter-Personal Notification" for a
 receipt notification and to "X.400 Inter-Personal Notification
 (failure)" for a non-receipt notification.
 Message-Type:
 Set to "InterPersonal Notification"
 References:
 Set to IPMS.IPN.subject-ipm
Kille Standards Track [Page 92]
RFC 2156 MIXER January 1998
 Discarded-X400-IPMS-Extensions:
 Used for any discarded IPN extensions.
 The following EBNF is defined for the body of the Message. This
 format is defined to ensure that all information from an
 interpersonal notification is available to the end user in a uniform
 manner.
 ipn-body-format = ipn-description <CRLF>
 [ ipn-extra-information <CRLF> ]
 [ ipn-content-return ]
 ipn-description = ipn-receipt / ipn-non-receipt
 ipn-receipt = "Your message to:" preferred-recipient <CRLF>
 "was received at" receipt-time <CRLF> <CRLF>
 "This notification was generated"
 acknowledgement-mode <CRLF>
 "The following extra information was given:" <CRLF>
 ipn-suppl <CRLF>
 ipn-non-receipt = "Your message to:"
 preferred-recipient <CRLF>
 ipn-reason
 ipn-reason = ipn-discarded / ipn-auto-forwarded
 ipn-discarded = "was discarded for the following reason:"
 discard-reason <CRLF>
 ipn-auto-forwarded = "was automatically forwarded." <CRLF>
 [ "The following comment was made:"
 auto-comment ]
 ipn-extra-information =
 "The following information types were converted:"
 encoded-info
 ipn-content-return = "The Original Message is not available"
 / "The Original Message follows:"
 preferred-recipient = mailbox
 receipt-time = date-time
 auto-comment = printablestring
 ipn-suppl = printablestring
Kille Standards Track [Page 93]
RFC 2156 MIXER January 1998
 discard-reason = "Expired" / "Obsoleted" /
 "User Subscription Terminated" / "IPM Deleted"
 acknowledgement-mode = "Manually" / "Automatically"
 The mappings for elements of the common fields of IPMS.IPN
 (IPMS.CommonFields) onto this structure and the message header are:
 subject-ipm
 Mapped to "References:"
 ipn-originator
 Mapped to "From:".
 ipn-preferred-recipient
 Mapped to EBNF.preferred-recipient
 conversion-eits
 Mapped to EBNF.encoded-info in EBNF.ipn-extra-information
 The mappings for elements of IPMS.IPN.non-receipt-fields
 (IPMS.NonReceiptFields) are:
 non-receipt-reason
 Used to select between EBNF.ipn-discarded and EBNF.ipn-auto-
 forwarded
 discard-reason
 Mapped to EBNF.discard-reason
 auto-forward-comment
 Mapped to EBNF.auto-comment
 returned-ipm
 This applies only to non-receipt notifications. EBNF.ipn-
 content-return shall always be omitted for receipt notifications,
 and always be present in non-receipt notifications. If present,
 the second option of EBNF.ipn-content-return is chosen, and the
 message is included. In this case, the message is formatted as
 multipart/mixed, and the returned message included as
 message/rfc822 after the text body part. Otherwise the first
 option is chosen.
 The mappings for elements of IPMS.IPN.receipt-fields
 (IPMS.ReceiptFields) are:
 receipt-time
 Mapped to EBNF.receipt-time
Kille Standards Track [Page 94]
RFC 2156 MIXER January 1998
 acknowledgement-mode
 Mapped to EBNF.acknowledgement-mode
 suppl-receipt-info
 Mapped to EBNF.ipn-suppl
 An example notification is:
 From: Steve Kille <steve@cs.ucl.ac.uk>
 To: Julian Onions <jpo@computer-science.nottingham.ac.uk>
 Subject: X.400 Inter-personal Notification
 Message-Type: InterPersonal Notification
 References: <1229.614418325@UK.AC.NOTT.CS>
 Date: Wed, 21 Jun 89 08:45:25 +0100
 Your message to: Steve Kille <steve@cs.ucl.ac.uk>
 was automatically forwarded.
 The following comment was made:
 Sent on to a random destination
 The following information types were converted: g3fax
5.3.6. Mappings from the MTS Abstract Service
 This section describes the MTS mappings for User Messages (IPM and
 IPN). This mapping is defined by specifying the mapping of
 MTS.MessageDeliveryEnvelope. The following extensions to RFC 822 are
 defined to support this mapping:
 mts-field = "X400-MTS-Identifier" ":" mts-msg-id
 / "X400-Originator" ":" mailbox
 / "X400-Recipients" ":" 1#mailbox
 / "Original-Encoded-Information-Types" ":"
 encoded-info
 / "X400-Content-Type" ":" mts-content-type
 / "X400-Content-Identifier" ":" printablestring
 / "Priority" ":" priority
 / "Originator-Return-Address" ":" 1#mailbox
 / "DL-Expansion-History" ":" mailbox ";" date-time
 ";"
 / "Conversion" ":" prohibition
 / "Conversion-With-Loss" ":" prohibition
 / "Delivery-Date" ":" date-time
 / "Discarded-X400-MTS-Extensions" ":"
 1#( object-identifier / labelled-integer )
 prohibition = "Prohibited" / "Allowed"
Kille Standards Track [Page 95]
RFC 2156 MIXER January 1998
 mts-msg-id = "[" global-id ";" *text "]"
 mts-content-type = "P2" / labelled-integer
 / object-identifier
 priority = "normal" / "non-urgent" / "urgent"
 The mappings for each element of MTS.MessageDeliveryEnvelope can now
 be considered. Where the specified action does not result in an
 extended element being mapped, the criticality associated with this
 element shall be considered. If the element is marked as critical
 for transfer or for delivery, the message shall be non delivered by
 the gateway because a critical extension cannot be correctly handled.
 MTS.MessageDeliveryEnvelope.message-delivery-identifier
 Mapped to the extended RFC 822 field "X400-MTS-Identifier:".
 MTS.MessageDeliveryEnvelope.message-delivery-time
 Discarded, as this time will be represented in an appropriate
 trace element.
 The mappings for elements of MTS.MessageDeliveryEnvelope.other-fields
 (MTS.OtherMessageDeliveryFields) are:
 content-type
 Mapped to the extended RFC 822 field "X400-Content-Type:". The
 string "P2" is retained for backwards compatibility with RFC 987.
 This shall not be generated, and either the EBNF.labelled-integer
 or EBNF.object-identifier encoding used.
 originator-name
 Mapped to the SMTP originator, and to the extended RFC 822 field
 "X400-Originator:". This is described in Section 4.6.2.
 original-encoded-information-types
 Mapped to the extended RFC 822 field "Original-Encoded-
 Information-Types:".
 priority
 Mapped to the extended RFC 822 field "Priority:".
 delivery-flags
 If the conversion-prohibited bit is set, add an extended RFC 822
 field "Conversion:".
 this-recipient-name and other-recipient-names
 The handling of these elements is described in Section 4.6.2.
Kille Standards Track [Page 96]
RFC 2156 MIXER January 1998
 originally-intended-recipient-name
 The handling of this element is described in Section 4.6.2.
 converted-encoded-information-types
 Discarded. This information will be mapped in the trace.
 message-submission-time
 Mapped to Date:.
 content-identifier
 Mapped to the extended RFC 822 field "X400-Content-Identifier:".
 In RFC 1327, this was "Content-Identifier:". This has been
 changed to avoid confusion with MIME defined fields. Gateways
 which reverse map, may support the old field.
 If any extensions (MTS.MessageDeliveryEnvelope.other-
 fields.extensions) are present, and they are marked as critical for
 transfer or delivery, then the message shall be rejected. The
 extensions (MTS.MessageDeliveryEnvelope.other-fields.extensions) are
 mapped as follows.
 conversion-with-loss-prohibited
 If set to MTS.ConversionWithLossProhibited.conversion-with-loss-
 prohibited, then add the extended RFC 822 field "Conversion-With-
 Loss:".
 requested-delivery-method
 Mapped to a comment, as described in Section 4.6.2.2.
 originator-return-address
 Mapped to the extended RFC 822 field "Originator-Return-Address:".
 physical-forwarding-address-request
 physical-delivery-modes
 registered-mail-type
 recipient-number-for-advice
 physical-rendition-attributes
 physical-delivery-report-request
 physical-forwarding-prohibited
 These elements are only appropriate for physical delivery.
 They are represented as comments in the "X400-Recipients:"
 field, as described in Section 4.6.2.2.
 originator-certificate
 message-token
 content-confidentiality-algorithm-identifier
Kille Standards Track [Page 97]
RFC 2156 MIXER January 1998
 content-integrity-check
 message-origin-authentication-check
 message-security-label
 proof-of-delivery-request
 These elements imply use of security services not available in the
 RFC 822 environment. If they are marked as critical for transfer
 or delivery, then the message shall be rejected. Otherwise they
 are discarded.
 redirection-history
 This is described in Section 4.6.2.
 dl-expansion-history
 Each element is mapped to an extended RFC 822 field "DL-
 Expansion-History:". These fileds shall be ordered in the message
 header, so that the most recent expansion comes first (same order
 as trace).
 If any MTS (or MTA) Extensions not specified in X.400 are present,
 and they are marked as critical for transfer or delivery, then the
 message shall be rejected. If they are not so marked, they can
 safely be discarded. The list of discarded fields shall be
 indicated in the extended header "Discarded-X400-MTS-Extensions:".
5.3.7. Mappings from the MTA Abstract Service
 There are some mappings at the MTA Abstract Service level which are
 done for IPM and IPN. These can be derived from
 MTA.MessageTransferEnvelope. The reasons for the mappings at this
 level, and the violation of layering are:
 - Allowing for multiple recipients to share a single RFC 822
 message
 - Making the X.400 trace information available on the RFC 822
 side
 - Making any information on deferred delivery available
 The SMTP recipients are calculated from the full list of X.400
 recipients. This is all of the members of
 MTA.MessageTransferEnvelope.per-recipient-fields being passed through
 the gateway, where the responsibility bit is set. In some cases, a
 different RFC 822 message would be calculated for each recipient, due
 to differing service requests for each recipient. As discussed in
 4.6.2.2, this specification allows either for multiple messages to be
 generated, or for the per-recipient information to be discarded.
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RFC 2156 MIXER January 1998
 The following EBNF is defined for extended RFC 822 headers:
 mta-field = "X400-Received" ":" x400-trace
 / "Deferred-Delivery" ":" date-time
 / "Latest-Delivery-Time" ":" date-time
 x400-trace = "by" md-and-mta ";"
 [ "deferred until" date-time ";" ]
 [ "converted" "(" encoded-info ")" ";" ]
 [ "attempted" md-or-mta ";" ]
 action-list
 ";" arrival-time
 md-and-mta = [ "mta" mta "in" ] global-id
 mta = word
 arrival-time = date-time
 md-or-mta = "MD" global-id
 / "MTA" mta
 Action-list = 1#action
 action = "Redirected"
 / "Expanded"
 / "Relayed"
 / "Rerouted"
 Note the EBNF.mta is encoded as 822.word. If the character set does
 not allow encoding as 822.atom, the 822.quoted-string encoding is
 used.
 If MTA.PerMessageTransferFields.deferred-delivery-time is present, it
 is used to generate a Deferred-Delivery: field. X.400 does not make
 this information available at the MTS level on delivery, because it
 requires that this service is provided by the first MTA. In the event
 that the first MTA does not provide this service, the function may
 optionally be implemented by the gateway: that is, the gateway may
 hold the message until the time specified in the protocol element.
 Thus, the value of this element will usually be in the past. For
 this reason, the extended RFC 822 field is primarily for information.
 If MTA.PerMessageTransferFields.extensions.dl-expansion-prohibited is
 present and set to dl-expansion-probited, the gateway may reject that
 message on the basis that it is unable to control distribution list
 expansion beyond the gateway. The service relating to this is
 described in Section 2.3.1.2. This approach was not specified in RFC
 1327. If it is found to be useful, it may be made mandatory in
 future versions of MIXER.
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RFC 2156 MIXER January 1998
 If MTA.PerMessageTransferFields.extensions.recipient-reassignment-
 prohibited is present and set to recipeint-reassignment-probited, the
 gateway may reject that message on the basis that it is unable to
 control distribution list expansion beyond the gateway. The service
 relating to this is described in Section 2.3.1.2. This approach was
 not specified in RFC 1327. If it is found to be useful, it may be
 made mandatory in future versions of MIXER.
 Merge MTA.PerMessageTransferFields.trace-information, and
 MTA.PerMessageTransferFields.internal-trace-information to produce a
 single ordered trace list. If Internal trace from other management
 domains has not been stripped, this may require complex interleaving.
 Where an element of internal trace and external trace are identical,
 except for the MTA in the internal trace, only the internal trace
 element shall be presented. Use this to generate a sequence of
 "X400-Received:" fields. The only difference between external trace
 and internal trace will be the extra MTA information in internal
 trace elements.
 When generating an RFC 822 message all trace fields (X400-Received
 and Received) shall be at the beginning of the header, before any
 other fields. Trace shall be in chronological order, with the most
 recent element at the front of the message. This ordering is
 determined from the order of the fields, not from timestamps in the
 trace, as there is no guarantee of clock synchronisation. A simple
 example trace (external) is:
 X400-Received: by /PRMD=UK.AC/ADMD=Gold 400/C=GB/ ; Relayed ;
 Tue, 20 Jun 89 19:25:11 +0100
 A more complex example (internal):
 X400-Received: by mta "UK.AC.UCL.CS" in
 /PRMD=UK.AC/ADMD=Gold 400/C=GB/ ;
 deferred until Tue, 20 Jun 89 14:24:22 +0100 ;
 converted (undefined, g3fax) ; attempted MD /ADMD=Foo/C=GB/ ;
 Relayed, Expanded, Redirected ; Tue, 20 Jun 89 19:25:11 +0100
 The gateway itself shall add a single line of trace information,
 indicating MIXER conversion by use of a comment. For example:
 Received: from isode.com by isode.com
 (MIXER Conversion following RFC 1327);
 Thu, 2 Jan 1997 14:46:03 +0000
 If SMTP is being used, Appendix A shall also be followed, which
 includes optional mappings to extension parameters.
Kille Standards Track [Page 100]
RFC 2156 MIXER January 1998
5.3.8. Mappings from Report Delivery
 that only reports destined for the MTS user will be mapped. Some
 additional services are also taken from the MTA service. X.400
 Delivery Reports are Mapped onto Delivery Status Notifications, as
 defined by NOTARY [28].
5.3.8.1. MTS Mappings
 A Delivery Report service will be represented as
 MTS.ReportDeliveryEnvelope, which comprises of per-report-fields
 (MTS.PerReportDeliveryFields) and per-recipient-fields.
 The enclosing message is a MIME message of content type
 multipart/report, with report-type=delivery-status, which is
 generated with the following fields:
 From:
 An administrator at the gateway system.
 To: A mapping of the
 MTA.ReportTransferEnvelope.report-destination-name. This is
 also the SMTP recipient.
 Message-Type:
 Set to "Delivery Report". This is strictly redundant, but
 retained for backwards compatibility with RFC 1327.
 Subject:
 The EBNF for the subject line is:
 subject-line = "Delivery-Report" "(" status ")"
 [ "for" destination ]
 status = "success" / "failure" / "success and failures"
 destination = mailbox / "MTA" word
 The subject is intended to give a clear indication as to the nature
 of the message, and summarise its contents. EBNF.status is set
 according to whether the recipients reported on are all successes,
 all failures, or a mixture. It is common for a report to reference a
 single recipient, in which case a subject line giving using all of
 the options of EBNF.status can be used. This gives useful
 information to the recipient. Where information varies between
 reported recpients, the options cannot be used. The EBNF.destination
 is used to indicate the addresses in the reports. If the report is
 for a single address, EBNF.mailbox is used to give the RFC 822
Kille Standards Track [Page 101]
RFC 2156 MIXER January 1998
 representation of the address. If all of the reported recpients
 reference the same MTA this is included in EBNF.word. The MTA is
 determined from the delivery report's trace.
 The format of the body of the message follows the NOTARY delivery
 status notification format, and is defined to ensure that all
 information is conveyed to the RFC 822 user in a consistent manner.
 The format is structured as if it was a message coming from the
 gateway, with three body parts. The first body part is ASCII text
 structured as follows:
 1. A few lines giving keywords to indicate the original
 message.
 2. A human summary of the status of each recipient being
 reported on.
 The second (mandatory) body part is the NOTARY delivery status
 notification, which contains detailed information extracted from the
 report. This information may be critical to diagnosing an obscure
 problem.
 The third (optional) body part contains the returned message (return
 of content). This structure is useful to the RFC 822 recipient, as
 it enables the original message to be extracted. For negative
 reports it shall be included if the original message is available.
 For positive reports headers from the message shall be included if
 the original message is available.
 The first body part containing the user oriented description is of
 type text/plain. The format of this body part is defined below as
 EBNF.dr-user-info.
 dr-user-info = dr-summary <CRLF>
 dr-recipients <CRLF>
 dr-content-return
 dr-content-return = "The Original Message is not available"
 / "The Original Message follows:"
 dr-summary = "This report relates to your message:" <CRLF>
 content-correlator <CRLF> <CRLF>
 "of" date-time <CRLF> <CRLF>
 dr-recipients = *(dr-recipient <CRLF> <CRLF>)
 dr-recipient = dr-recip-success / dr-recip-failure
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RFC 2156 MIXER January 1998
 dr-recip-success =
 "Your message was successfully delivered to:"
 mailbox "at" date-time
 dr-recip-failure = "Your message was not delivered to:"
 mailbox <CRLF>
 "for the following reason:" *word report-point
 = [ "mta" mta-name "in" ] global-id content-correlator = *word
 mta-name = word
 EBNF.dr-summary
 The EBNF.content-correlator is taken from the content correlator
 (or content identifier if there is no content correlator) and the
 EBNF.date-time from the trace, as described in Section 5.3.8.3.
 LWSP may be added to improve the layout of the body part.
 EBNF.dr-recipients
 There is an element for each recipient in the delivery report. In
 each case, EBNF.mailbox is taken from the RFC 822 form of the
 originally specified recipient, which is taken from the originally
 specified recipient element if present or from the actual
 recipient. When reporting success, the message delivery time is
 used to derive EBNF.date-time. When reporting failure, the
 information includes a human readable interpretation of the X.400
 diagnostic and reason codes, and the supplementary information.
 EBNF.dr-content-return
 This is set according to whether or not the content is being
 returned.
 The EBNF of this body part is designed for english-speaking users.
 The language of the strings in the EBNF may be altered.
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RFC 2156 MIXER January 1998
 The EBNF used in the delivery status notification is:
 dr-per-message-fields =
 / "X400-Conversion-Date" ":" date-time
 / "X400-Subject-Submision-Identifier" ":"
 mts-msg-id
 / "X400-Content-Identifier" ":" printablestring
 / "X400-Content-Type" ":" mts-content-type
 / "X400-Original-Encoded-Information-Types" ":"
 encoded-info
 / "X400-Originator-and-DL-Expansion-History" ":"
 mailbox ";" date-time ";"
 / "X400-Reporting-DL-Name" ":" mailbox
 / "X400-Content-Correlator" ":" content-correlator
 / "X400-Recipient-Info" ":" recipient-info
 / "X400-Subject-Intermediate-Trace-Information" ":"
 x400-trace
 / dr-extensions
 dr-per-recipient-fields =
 / "X400-Redirect-Recipient" ":" "x400" ";" std-or
 / "X400-Mapped-Redirect-Recipient" ":" "rfc822" ";" mailbox
 / "X400-Converted-EITs" ":" encoded-info ";"
 / "X400-Delivery-Time" ":" date-time
 / "X400-Type-of-MTS-User" ":" labelled-integer
 / "X400-Last-Trace" ":" [ encoded-info ] date-time
 / "X400-Supplementary-Info" ":"
 <"> printablestring <"> ";"
 / "X400-Redirection-History" ":" redirect-history-item
 / "X400-Physical-Forwarding-Address" ":" mailbox
 / "X400-Originally-Specified-Recipient-Number" ":"
 integer
 / dr-extensions
 dr-extensions = "X400-Discarded-DR-Extensions" ":"
 1# (object-identifier / labelled-integer)
 dr-diagnostic = "Reason" labelled-integer-2
 [ ";" "Diagnostic" labelled-integer-2 ]
 A body part of type delivery status, as defined by NOTARY, is
 generated. MIXER extends this delivery status notification (DSN)
 specification, by defining additional per message fields in EBNF.dr-
 per-message-fields and additional per recipient fields in EBNF.dr-
 per-recipient-fields. These are used as extensions to DSN.per-
 message-fields and DSN.per-recipient-fields. MIXER also defines a
 new NOTARY address type "x400", with encoding of EBNF.std-or. A
 directory name may be inluded as an RFC 822 comment.
Kille Standards Track [Page 104]
RFC 2156 MIXER January 1998
 The following DSN.per-message-fields are always generated:
 DSN.reporting-mta-field
 The DSN.mta-name-type is set to "x400", and this string is
 reserved by MIXER. The DSN.mta-name has its syntax specified by
 EBNF.report-point, with the information derived from the first
 element of the DR's trace.
 DSN.arrival-date-field
 This is derived from the date of the
 MTA.PerRecipientReportTransferFields.last-trace-info.arrival-time
 of the first recipient in the report.
 The following two EBNF.per-message-fields are generated by the MIXER
 gateway:
 DSN.dsn-gateway-field
 The type is set to "dns" and the domain set to the local domain
 of the gateway.
 X400-Conversion-Date:
 The EBNF.date-time is set to the time of the MIXER conversion.
 The elements of MTS.ReportDeliveryEnvelope.per-report-fields are
 mapped as follows onto the DSN per message fields as follows:
 subject-submission-identifier
 Mapped to DSN.original-envelope-id-field. The encoding of this
 MTS Identifier follows the format EBNF.mts-msg-id.
 content-identifier
 Mapped to X400-Content-Identifier:
 content-type
 Mapped to X400-Content-Type:
 original-encoded-information-types
 Mapped to X400-Encoded-Info:
 The extensions from MTS.ReportDeliveryEnvelope.per-report-
 fields.extensions are mapped as follows:
 originator-and-DL-expansion-history
 Each element is mapped to an "X400-Originator-and-DL-Expansion-
 History:" They shall be ordered so that the most recent expansion
 comes first in the header (same order as trace).
Kille Standards Track [Page 105]
RFC 2156 MIXER January 1998
 reporting-DL-name
 Mapped to X400-Reporting-DL-Name:
 content-correlator
 If the content correlator starts with the string "SMTP/NOTARY
 ENVID: ", then the remainder of the content correlator is mapped
 to the DSN original-envelope-id field. If this is not the case,
 the content correlator is mapped to X400-Content-Correlator:,
 provided that the encoding is IA5String (this will always be the
 case).
 message-security-label
 reporting-MTA-certificate
 report-origin-authentication-check
 These security parameters will not be present unless there is an
 error in a remote MTA. If they are present, they shall be
 discarded in preference to discarding the whole report. They
 shall be listed in the X400-Discarded-DR-Extensions: field.
 If there are any other DR extensions, they shall also be discarded
 and listed in the X400-Discarded-DR-Extensions: field.
 For each element of MTS.ReportDeliveryEnvelope.per-recipient-fields,
 a set of DSN.per-recipient-fields is generated. The fields are
 filled in as follows:
 actual-recipient-name
 If originally-intended-recipient-name is not present, generate a
 DSN.original-recipient-field fields, with DSN.address-type of
 "rfc822", and with an RFC 822 mailbox generated from the address
 encoded as specified by NOTARY. Also generate a DSN.final-
 recipient-field field, which holds the X.400 representation of the
 same address. If the directory name is present, it shall be added
 as a trailing comment in the X.400 form.
 If originally-intended-recipient-name is present, generate an
 "X400-Mapped-Redirect-Recipient:" field, with DSN.address-type of
 "rfc822", and with an RFC 822 mailbox generated from the address
 encoded as specified by NOTARY. Also generate an "X400-Redirect-
 Recipient:" field, which holds the X.400 representation of the
 same address. If the directory name is present, it shall be added
 as a trailing comment in the X.400 form.
Kille Standards Track [Page 106]
RFC 2156 MIXER January 1998
 report
 If it is MTS.Report.delivery, then set DSN.action-field to
 "delivered", and set "X400-Delivery-Time:" and "X400-Type-of-MTS-
 User:" from the information in the report. DSN.status field is
 set to "2.0.0".
 If it is MTS.Report.non-delivery, then set DSN.action-field to
 "failed". DSN.diagnostic-code-field is encoded according to the
 syntax EBNF.dr-diagnostic, with the labelled integers set from the
 reason and diagnostic codes. DSN.status-field is derived from the
 reason and diagnostic codes, as described below.
 converted-encoded-information-types
 Set X400-Converted-EITs:
 originally-intended-recipient
 Generate a DSN.final-recipient-field field, with DSN.address-type
 of "rfc822", and with an RFC 822 mailbox generated from the
 address encoded as specified by NOTARY. Also generate a
 DSN.original-recipient-field field, which holds the X.400
 representation of the same address. If the directory name is
 present, it shall be added as a trailing comment in the X.400
 form.
 supplementary-info
 Set X400-Supplementary-Info:
 redirection-history
 Generate an "X400-Redirection-History:" field for each redirect
 history element. The fields are ordered with the earliest
 redirect first.
 physical-forwarding-address
 Set X400-Physical-Forwarding-Address as a mailbox, with directory
 name in comment if present.
 recipient-certificate
 Discard
 proof-of-delivery
 Discard
 Any unknown extensions shall be discarded, irrespective of
 criticality. All discarded extensions shall be included in a "X400-
 Discarded-DR-Extensions:" field.
Kille Standards Track [Page 107]
RFC 2156 MIXER January 1998
 The number from the MTA.PerRecipientReportTransferFields.originally-
 specified-recipient-number shall be mapped to "X400-Originally-
 Specified-Recipient-Number:", in order to facilitate reverse mapping
 of delivery reports.
 The original message shall be included in the delivery status
 notification if it is available. The original message will usually be
 available at the gateway, as discussed in Section 5.2. If the
 original message is available, but is not a legal message format, a
 dump of the ASN.1 may be included, encoded as application/octet-
 string. This is recommended, but not required.
 Where the original message is included, it shall be encoded according
 to the MIME specifications as content type message/rfc822.
5.3.8.2. Status Code Mappings
 This section defines the mappings from X.400 diagnostic and status
 codes to the NOTARY Status field.
C/D X400 meaning DSN code Means
0/Any Transfer failure (may be temporary) 4.4.0 Other net/route
1/Any Unable to transfer 5.0.0 Other, unknown
2/Any Conversion not performed 5.6.3 Conv not supported
3/Any Physical rendition not performed 5.6.0 Other media error
4/Any Physical delivery not performed 5.1.0 Other address
 status
5/Any Restricted delivery 5.7.1
6/Any Directory operation unsuccessful 5.4.3 Routing server
 failure
7/Any Deferred delivery not performed 5.3.3 Not capable
1/0 Unrecognized OR name 5.1.1
1/1 Ambiguous OR name 5.1.4
1/2 MTS congestion 4.3.1
1/3 Loop detected 5.4.6
1/4 Recipient unavailable 4.2.1
1/5 Delivery time expired 4.4.7
1/6 Encoded information types unsupported 5.6.1 Media unsupp.
1/7 Content too long 5.2.3
2/8 Conversion impractical 5.6.3
2/9 Conversion prohibited 5.6.3
1/10 Implicit conversion not subscribed 5.6.3
1/11 Invalid arguments 5.5.2
1/12 Content syntax error 5.5.2
1/13 Size constraint violation 5.5.2
1/14 Protocol violation 5.5.0
Kille Standards Track [Page 108]
RFC 2156 MIXER January 1998
1/15 Content type not supported 5.6.1 Media unsupp.
1/16 Too many recipients 5.5.3
1/17 No bilateral agreement 5.4.4
1/18 Unsupported critical function 5.3.3 System not capable
2/19 Conversion with loss prohibited 5.6.2
2/20 Line too long 5.6.0
2/21 Page split 5.6.0
2/22 Pictorial symbol loss 5.6.2
2/23 Punctuation symbol loss 5.6.2
2/24 Alphabetic character loss 5.6.2
2/25 Multiple information loss 5.6.2
1/26 Recipient reassignment prohibited 5.4.0 Undefined net/route
1/27 Redirection loop detected 5.4.6
1/28 DL expansion prohibited 5.7.2
1/29 No DL submit permission 5.7.1 Delivery not
 authorized
1/30 DL expansion failure 4.2.4
4/31 Physical rendition attrs not supported 5.6.0 Undefined media
 error
4/32-45 Various physical mail stuff 5.1.0 Other address
 status
1/43 New address unknown 5.1.6 Destination mbox
 moved
1/46 Secure messaging error 5.7.0 Other security
 status
2/47 Unable to downgrade 5.3.3 System not capable
0/48 Unable to complete transfer 5.3.4 Message too big
0/49 Transfer attempts limit reached 4.4.7 Delivery time
 expired
5.3.8.3. MTA Mappings
 The single SMTP recipient is constructed from
 MTA.ReportTransferEnvelope.report-destination-name, using the
 mappings of Chapter 4. Unlike with a user message, this information
 is not available at the MTS level.
 The following additional mappings are made, which results in fields
 in the outer header of the DSN.
 MTA.ReportTransferEnvelope.report-destination-name
 This is used to generate the To: field.
 MTA.ReportTransferEnvelope.identifier
 Mapped to the extended RFC 822 field "X400-MTS-Identifier:". It
 may also be used to derive a "Message-Id:" field.
Kille Standards Track [Page 109]
RFC 2156 MIXER January 1998
 MTA.ReportTransferEnvelope.trace-information
 and
 MTA.ReportTransferEnvelope.internal-trace-information
 Mapped onto the extended RFC 822 field "X400-Received:", as
 described in Section 5.3.7. Date: is generated from the first
 element of trace.
 The following additional mappings are made, which result in per
 message fields in the DSN body part:
 MTA.PerRecipientReportTransferFields.last-trace-information
 Mapped to X400-Last-Trace:".
 MTA.PerReportTransferFields.subject-intermediate-trace-
 information Mapped to "X400-Subject-Intermediate-Trace-
 Information:". These fields are ordered so that the most recent
 trace element comes first.
5.3.8.4. Example Delivery Reports
 This section contains sample delivery reports. These are the same
 examples used in RFC 1327, and so they also illustrate the changes
 between RFC 1327 and this document. Example Delivery Report 1:
 Received: from cs.ucl.ac.uk by bells.cs.ucl.ac.uk
 via Delivery Reports Channel id <27699-0@bells.cs.ucl.ac.uk>;
 Thu, 7 Feb 1991 15:48:39 +0000 From: UCL-CS MTA
 <postmaster@cs.ucl.ac.uk> To: S.Kille@cs.ucl.ac.uk Subject: Delivery
 Report (failure) for H.Hildegard@bbn.com Message-Type: Delivery
 Report Date: Thu, 7 Feb 1991 15:48:39 +0000 Message-ID:
 <"bells.cs.u.694:07.01.91.15.48.34"@cs.ucl.ac.uk> X400-Content-
 Identifier: Greetings. MIME-Version: 1.0 Content-Type:
 multipart/report; report-type=delivery-status;
 boundary=boundary-1
 --boundary-1
 This report relates to your message:
 Greetings.
 of Thu, 7 Feb 1991 15:48:20 +0000
 Your message was not delivered to
 H.Hildegard@bbn.com for the following reason:
 Bad Address
 MTA 'bbn.com' gives error message (USER) Unknown user name
 in
Kille Standards Track [Page 110]
RFC 2156 MIXER January 1998
 "H.Hildegard@bbn.com"
 The Original Message follows:
 --boundary-1 content-type: message/delivery-status
 Reporting-MTA: x400; bells.cs.ucl.ac.uk in /PRMD=uk.ac/ADMD=gold
 400/C=gb/ Arrival-Date: Thu, 7 Feb 1991 15:48:34 +0000 DSN-Gateway:
 dns; bells.cs.ucl.ac.uk X400-Conversion-Date: Thu, 7 Feb 1991
 15:48:40 +0000 Original-Envelope-Id:
 [/PRMD=uk.ac/ADMD=gold
 400/C=gb/;<1803.665941698@UK.AC.UCL.CS>] X400-Content-Identifier:
 Greetings. X400-Subject-Intermediate-Trace-Information:
 /PRMD=uk.ac/ADMD=gold 400/C=gb/;
 arrival Thu, 7 Feb 1991 15:48:20 +0000 action Relayed X400-
 Subject-Intermediate-Trace-Information: /PRMD=uk.ac/ADMD=gold
 400/C=gb/;
 arrival Thu, 7 Feb 1991 15:48:18 +0000 action Relayed
 Original-Recipient: rfc822; H.Hildegard@bbn.com Final-Recipient:
 x400;
 /RFC-822=H.Hildegard(a)bbn.com/OU=cs/O=ucl/PRMD=uk.ac/ADMD=gold
 400/C=gb/; Action: failure Status: 5.1.1 Diagnostic-Code: x400;
 Reason 1 (Unable-To-Transfer);
 Diagnostic 0 (Unrecognised-ORName) X400-Last-Trace: (ia5) Thu, 7
 Feb 1991 15:48:18 +0000; X400-Originally-Specified-Recipient-Number:
 1 X400-Supplementary-Info: "MTA 'bbn.com' gives error message (USER)
 Unknown user name in "H.Hildegard@bbn.com"";
 --boundary-1 Content-Type: message/rfc822
 Received: from glenlivet.cs.ucl.ac.uk by bells.cs.ucl.ac.uk
 with SMTP inbound id <27689-0@bells.cs.ucl.ac.uk>;
 Thu, 7 Feb 1991 15:48:21 +0000 To: H.Hildegard@bbn.com Subject:
 Greetings. Phone: +44-71-380-7294 Date: Thu, 07 Feb 91 15:48:18
 +0000 Message-ID: <1803.665941698@UK.AC.UCL.CS> From: Steve Kille
 <S.Kille@cs.ucl.ac.uk>
 Steve
 --boundary-1--
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RFC 2156 MIXER January 1998
 Example Delivery Report 2:
 Received: from cs.ucl.ac.uk by bells.cs.ucl.ac.uk
 via Delivery Reports Channel id <27718-0@bells.cs.ucl.ac.uk>;
 Thu, 7 Feb 1991 15:49:11 +0000
 X400-Received: by mta "bells.cs.ucl.ac.uk" in
 /PRMD=uk.ac/ADMD=gold 400/C=gb/;
 Relayed; Thu, 7 Feb 1991 15:49:08 +0000
 X400-Received: by /PRMD=DGC/ADMD=GOLD 400/C=GB/; Relayed;
 Thu, 7 Feb 1991 15:48:40 +0000
 From: UCL-CS MTA <postmaster@cs.ucl.ac.uk>
 To: S.Kille@cs.ucl.ac.uk
 Subject: Delivery Report (failure) for
 j.nosuchuser@dle.cambridge.DGC.gold-400.gb
 Message-Type: Delivery Report
 Date: Thu, 7 Feb 1991 15:46:11 +0000
 Message-ID: <"DLE/910207154840Z/000"@cs.ucl.ac.uk>
 X400-Content-Identifier: A useful mess...
 MIME-Version: 1.0
 Content-Type: multipart/report; report-type=delivery-status;
 boundary=boundary-1
 --boundary-1
 This report relates to your message:
 A useful mess...
 of Thu, 7 Feb 1991 15:43:20 +0000
 Your message was not delivered to
 j.nosuchuser@dle.cambridge.DGC.gold-400.gb
 for the following reason:
 Bad Address
 DG 21187: (CEO POA) Unknown addressee.
 The Original Message is not available
 --boundary-1
 content-type: message/delivery-status
 Reporting-MTA: x400; /PRMD=DGC/ADMD=GOLD 400/C=GB/
 Arrival-Date: Thu, 7 Feb 1991 15:48:40 +0000
 DSN-Gateway: dns; bells.cs.ucl.ac.uk
 X400-Conversion-Date: Thu, 7 Feb 1991 15:49:12 +0000
 Original-Envelope-Id:
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 [/PRMD=uk.ac/ADMD=gold 400/C=gb/;<1796.665941626@UK.AC.UCL.CS>]
 X400-Content-Identifier: A useful mess...
 Original-Recipient: rfc822; j.nosuchuser@dle.cambridge.DGC.gold-400.gb
 Final-Recipient: x400;
 /I=j/S=nosuchuser/OU=dle/O=cambridge/PRMD=DGC/ADMD=GOLD 400/C=GB/
 Action: failure
 Status: 5.1.1
 Diagnostic-Code: x400; Reason 1 (Unable-To-Transfer);
 Diagnostic 0 (Unrecognised-ORName)
 X400-Supplementary-Info: "DG 21187: (CEO POA) Unknown addressee."
 X400-Originally-Specified-Recipient-Number: 1
 --boundary-1--
5.3.9. Probe
 This is an MTS internal issue. Any probe shall be serviced by the
 gateway, as there is no equivalent RFC 822 functionality. The value
 of the reply is dependent on whether the gateway could service an MTS
 Message with the values specified in the probe. The reply shall make
 use of MTS.SupplementaryInformation to indicate that the probe was
 serviced by the gateway.
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Appendix A - Mappings Specific to SMTP
 This Appendix is specific to the Simple Mail Transfer Protocol (RFC
 821). It describes specific changes in the context of this protocol.
 When MIXER is used with SMTP, conformance to this appendix is
 mandatory.
 1. Probes
 When servicing a probe, as described in section 5.3.9, use may be
 made of the SMTP VRFY command to increase the accuracy of information
 contained in the delivery report.
 2. Long Lines
 SMTP is a text oriented protocol, and is required to support a line
 length of at least 1000 characters. Some implementations do not
 support line lengths greater than 1000 characters. This can cause
 problems. Where body parts have long lines, it is recommended to use
 a MIME encoding that folds lines (quoted printable).
 3. SMTP Extensions
 There are several RFCs that specify extensions to SMTP. Most of these
 are not relevant to MIXER. The NOTARY work to support delivery
 report defines extensions which are relevant [29]. Use of these
 extensions by a MIXER gateway is optional. If these extensions are
 used, they shall be used in the manner described below.
 3.1. SMTP Extension mapping to X.400
 Mappings are defined for the following extensions:
 NOTIFY
 This is used to set the report and non-delivery bits of
 MTA.PerRecipientMessageTransferFields.per-recipient-indicators.
 If the value is NEVER, both bits are zero. If SUCCESS is present,
 the report bit is set. Otherwise, the non-delivery-report bit is
 set. If the gateway uses the NOTIFY command, it shall perform
 this mapping in all cases.
 ORCPT
 If the address type of the original recipient is "x400" or
 "rfc822", this may be used at the MTS level, to generate an
 element of redirection history, with the redirection date being
 the date of conversion and the reason set to "alias".
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 ENVID
 If present, this may be used to generate a content correlator.
 This is used rather than the MTS Identifier, as the ENVID is
 unique for the UA only and is likely to be too large to map to an
 MTS identifier. The content correlator is encoded as an IA5 String
 containing the ENVID and prefixed by the string:
 "SMTP/NOTARY ENVID: "
 If the ENVID starts with the string "X400-MTS-Identifier: ", then
 this ENVID was generated from an X.400 MTS Identifier. The
 reverse mapping defined in Section 3.2 of Appendix A shall not be
 used, as this may cause problems in certain situations (e.g.,
 where the message was expanded by an Internet mailing list).
 3.2. X.400 Mapping to SMTP Extensions
 The following extensions may be used as a part of the MIXER mapping:
 NOTIFY
 The originator-report and originator-non-delivery-report bits of
 MTA.PerRecipientMessageTransferFields.per-recipient-indicators
 determine how this is used. If both bits are zero, the parameter
 is NEVER. If the report bit is set, SUCCESS is used. Otherwise,
 FAILURE is used. If this is done, the gateway shall not generate
 a delivery report for this recipient, unless this is needed in the
 case where the originating MTA service report requirements differ
 from the user requirements. Additional originating MTA
 requrirements are satisfied by the gateway.
 ORCPT
 If the MTS.perRecipientDeliveryFields.originally-intended-
 recipient-name is present, the ORCPT command may be used to carry
 this value, using the "x400" syntax.
 ENVID
 This may be generated, with the value taken from the
 MTS.MessageDeliveryEnvelope.message-delivery-identifer. If this
 is done, it shall be encoded as EBNF.mts-msg-id, preceded by the
 string "X400-MTS-Identifier: ".
 RET
 If MTA.PerMessageTransferFields.per-message-indicators.content-
 return-request is set to FALSE, the parameter RET may be set to
 HDRS, to specify return of headers only.
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Appendix B - Mapping with X.400(1984)
 This appendix defines modifications to the mapping for use with
 X.400(1984).
 The X.400(1984) protocols are a proper subset of X.400(1988). When
 mapping from X.400(1984) to RFC 822, no changes to this specification
 are needed.
 When mapping from RFC 822 to X.400(1984), no use can be made of 1988
 specific features. No use of such features is made at the MTS
 level. The heading extension feature is used at the IPMS level, and
 this shall be replaced by the RFC 987 approach. All header
 information which would usually be mapped into the rfc-822-heading-
 list extension is mapped into a single IA5 body part, which is the
 first body part in the message. This body part will start with the
 string "RFC-822-Headers:" as the first line. The headers then follow
 this line. This specification requires correct reverse mapping of
 this format, either from 1988 or 1984. RFC 822 extended headers
 which could be mapped into X.400(1988) elements, are also mapped to
 the body part.
 In an environment where RFC 822 is of major importance, it may be
 desirable for downgrading to consider the case where the message was
 originated in an RFC 822 system, and mapped according to this
 specification. The rfc-822-heading-list extension may be mapped
 according to this appendix.
 When parsing std-or, the following restrictions shall be observed:
 - Only the 84/88 attributes identified in the table in
 Section 4.2 are present.
 - No teletex encoding is allowed.
 If an address violates this, it shall be treated as an RFC 822
 address, which will usually lead to encoding as a DDA "RFC-822".
 It is possible that attributes of zero length may be present
 in an OR Address. This is not legal in 1988, except for ADMD
 where the case is explicitly described in Section 4.3.5.
 Attributes of zero length are deprecated (the attribute shall be
 omitted), and will therefore be unusual. However, some systems
 generate them and rely on them. Therefore, any null attribute
 shall be enoded using the std-or encoding (e.g., /O=/).
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 If a non-Teletex Common Name (CN) is present, it shall be
 mapped onto a Domain Defined Attribute "Common". This is in line
 with RFC 1328 on X.400 1988 to 1984 downgrading [22].
 This specification defines a mapping of the Internet message
 framework to X.400. Body part mappings are defined in RFC
 2157 [6], which relies on X.400(88) features. Downgrading to
 X.400(84) for body parts is defined in RFC 1496 (HARPOON), which
 shall be followed in the context of this appendix [5].
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Appendix C - RFC 822 Extensions for X.400 access
 This appendix defines a number of optional mappings which may be
 provided to give access from RFC 822 to a number of X.400 services.
 These mappings are beyond the basic scope of this specification.
 There has been a definite demand to use extended RFC 822 as a
 mechanism to access X.400, and these extensions provide access to
 certain features. If this functionality is provided, this appendix
 shall be followed. The following headings are defined:
 extended-heading =
 "Prevent-NonDelivery-Report" ":"
 / "Generate-Delivery-Report" ":"
 / "Alternate-Recipient" ":" prohibition
 / "Disclose-Recipients" ":" prohibition
 / "X400-Content-Return" ":" prohibition
 Prevent-NonDelivery-Report and Generate-Delivery-Report allow setting
 of MTS.PerRecipientSubmissionFields.originator-report-request. The
 setting will be the same for all recipients.
 Alternate-Recipient, Disclose-Recipients, and X400-Content-Return
 allow for override of the default settings for
 MTS.PerMessageIndicators.
 Use of NOTARY mechanisms is a preferred meachanism for controlling
 these parameters.
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Appendix D - Object Identifier Assignment
 The following Object Identifiers shall be used.
 internet ::= OBJECT IDENTIFIER { iso org(3) dod(6) 1 } -- from RFC
 1155
 mail OBJECT IDENTIFIER ::= { internet 7 } -- IANA assigned
 mixer OBJECT IDENTIFIER ::= { mail mixer(1) } -- inherited from RFC
 1495
 mixer-core OBJECT IDENTIFIER ::= { mixer core(3) }
 id-rfc-822-field-list OBJECT IDENTIFIER ::= {mixer-core 2}
 id-dsn-header-list OBJECT IDENTIFIER ::= {mixer-core 3}
 id-dsn-field-list OBJECT IDENTIFIER ::= {mixer-core 4}
 eit-mixer OBJECT IDENTIFIER ::= {mixer-core 5}
 -- the MIXER pseudo-EIT
 This object identifier for id-rfc-822-field-list is different to
 the one assigned in RFC 1327, which was erroneous.
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Appendix E - BNF Summary
 boolean = "TRUE" / "FALSE"
 numericstring = *(DIGIT / " ")
 printablestring = *( ps-char )
 ps-restricted-char = 1DIGIT / 1ALPHA / " " / "'" / "+"
 / "," / "-" / "." / "/" / ":" / "=" / "?"
 ps-delim = "(" / ")"
 ps-char = ps-delim / ps-restricted-char
 ps-encoded = *( ps-restricted-char / ps-encoded-char )
 ps-encoded-char = "(a)" ; (@)
 / "(p)" ; (%)
 / "(b)" ; (!)
 / "(q)" ; (")
 / "(u)" ; (_)
 / "(l)" ; "("
 / "(r)" ; ")"
 / "(" 3DIGIT ")"
 teletex-string = *( ps-char / t61-encoded )
 t61-encoded = "{" 1* t61-encoded-char "}"
 t61-encoded-char = 3DIGIT
 teletex-and-or-ps = [ printablestring ] [ "*" teletex-string ]
 labelled-integer ::= [ key-string ] "(" numericstring ")"
 labelled-integer-2 ::= [ numericstring ] "(" key-string ")"
 key-string = *key-char
 key-char = <a-z, A-Z, 0-9, and "-">
 object-identifier ::= oid-comp object-identifier
 | oid-comp
 oid-comp ::= [ key-string ] "(" numericstring ")"
 encoded-info = 1#encoded-type
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 encoded-type = built-in-eit / object-identifier
 built-in-eit = "Undefined" ; undefined (0)
 / "Telex" ; tLX (1)
 / "IA5-Text" ; iA5Text (2)
 / "G3-Fax" ; g3Fax (3)
 / "TIF0" ; tIF0 (4)
 / "Teletex" ; tTX (5)
 / "Videotex" ; videotex (6)
 / "Voice" ; voice (7)
 / "SFD" ; sFD (8)
 / "TIF1" ; tIF1 (9)
 encoded-pn = [ given "." ] *( initial "." ) surname
 given = 2*<ps-char not including ".">
 initial = ALPHA
 surname = printablestring
 std-or-address = 1*( "/" attribute "=" value ) "/"
 attribute = standard-type
 / "RFC-822"
 / dd-key "." std-printablestring
 std-or-address-input = [ sep pair ] sep pair *( sep pair )
 sep [ pair sep ]
 sep = "/" / ";"
 pair = input-attribute "=" value
 input-attribute = attribute
 / dd-key ":" std-printablestring
 standard-type = key-string
 dd-key = key-string
 value = std-printablestring
 std-printablestring
 = *( std-char / std-pair )
 std-char = <"{", "}", "*", and any ps-char
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 except "/" and "=" >
 std-pair = "$" ps-char
 global-id = std-or-address
 mta-field = "X400-Received" ":" x400-trace
 / "Deferred-Delivery" ":" date-time
 / "Latest-Delivery-Time" ":" date-time
 x400-trace = "by" md-and-mta ";"
 [ "deferred until" date-time ";" ]
 [ "converted" "(" encoded-info ")" ";" ]
 [ "attempted" md-or-mta ";" ]
 action-list
 ";" arrival-time
 md-and-mta = [ "mta" mta "in" ] global-id
 mta = word
 arrival-time = date-time
 md-or-mta = "MD" global-id
 / "MTA" mta
 Action-list = 1#action
 action = "Redirected"
 / "Expanded"
 / "Relayed"
 / "Rerouted"
 dr-user-info = dr-summary <CRLF>
 dr-recipients <CRLF>
 dr-content-return
 dr-content-return = "The Original Message is not available"
 / "The Original Message follows:"
 dr-summary = "This report relates to your message:" <CRLF>
 content-correlator <CRLF> <CRLF>
 "of" date-time <CRLF> <CRLF>
 dr-recipients = *(dr-recipient <CRLF> <CRLF>)
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 dr-recipient = dr-recip-success / dr-recip-failure
 dr-recip-success =
 "Your message was successfully delivered to:"
 mailbox "at" date-time
 dr-recip-failure = "Your message was not delivered to:"
 mailbox <CRLF>
 "for the following reason:" *word
 report-point = [ "mta" mta-name "in" ] global-id
 content-correlator = *word
 mta-name = word
 dr-per-message-fields =
 / "X400-Conversion-Date" ":" date-time
 / "X400-Subject-Submision-Identifier" ":"
 mts-msg-id
 / "X400-Content-Identifier" ":" printablestring
 / "X400-Content-Type" ":" mts-content-type
 / "X400-Original-Encoded-Information-Types" ":"
 encoded-info
 / "X400-Originator-and-DL-Expansion-History" ":"
 mailbox ";" date-time ";"
 / "X400-Reporting-DL-Name" ":" mailbox
 / "X400-Content-Correlator" ":" content-correlator
 / "X400-Recipient-Info" ":" recipient-info
 / "X400-Subject-Intermediate-Trace-Information" ":"
 x400-trace
 / dr-extensions
 dr-per-recipient-fields =
 / "X400-Redirect-Recipient" ":" "x400" ";" std-or
 / "X400-Mapped-Redirect-Recipient" ":" "rfc822" ";"
 mailbox
 / "X400-Converted-EITs" ":" encoded-info ";"
 / "X400-Delivery-Time" ":" date-time
 / "X400-Type-of-MTS-User" ":" labelled-integer
 / "X400-Last-Trace" ":" [ encoded-info ] date-time
 / "X400-Supplementary-Info" ":"
 <"> printablestring <"> ";"
 / "X400-Redirection-History" ":" redirect-history-item
 / "X400-Physical-Forwarding-Address" ":" mailbox
 / "X400-Originally-Specified-Recipient-Number" ":"
 integer
 / dr-extensions
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 dr-extensions = "X400-Discarded-DR-Extensions" ":"
 1# (object-identifier / labelled-integer)
 dr-diagnostic = "Reason" labelled-integer-2
 [ ";" "Diagnostic" labelled-integer-2 ]
 mts-field = "X400-MTS-Identifier" ":" mts-msg-id
 / "X400-Originator" ":" mailbox
 / "X400-Recipients" ":" 1#mailbox
 / "Original-Encoded-Information-Types" ":"
 encoded-info
 / "X400-Content-Type" ":" mts-content-type
 / "X400-Content-Identifier" ":" printablestring
 / "Priority" ":" priority
 / "Originator-Return-Address" ":" 1#mailbox
 / "DL-Expansion-History" ":" mailbox ";" date-time ";"
 / "Conversion" ":" prohibition
 / "Conversion-With-Loss" ":" prohibition
 / "Delivery-Date" ":" date-time
 / "Discarded-X400-MTS-Extensions" ":"
 1#( object-identifier / labelled-integer )
 prohibition = "Prohibited" / "Allowed"
 mts-msg-id = "[" global-id ";" *text "]"
 mts-content-type = "P2" / labelled-integer
 / object-identifier
 priority = "normal" / "non-urgent" / "urgent"
 ipn-body-format = ipn-description <CRLF>
 [ ipn-extra-information <CRLF> ]
 [ ipn-content-return ]
 ipn-description = ipn-receipt / ipn-non-receipt
 ipn-receipt = "Your message to:" preferred-recipient <CRLF>
 "was received at" receipt-time <CRLF> <CRLF>
 "This notification was generated"
 acknowledgement-mode <CRLF>
 "The following extra information was given:" <CRLF>
 ipn-suppl <CRLF>
 ipn-non-receipt = "Your message to:"
 preferred-recipient <CRLF>
 ipn-reason
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 ipn-reason = ipn-discarded / ipn-auto-forwarded
 ipn-discarded = "was discarded for the following reason:"
 discard-reason <CRLF>
 ipn-auto-forwarded = "was automatically forwarded." <CRLF>
 [ "The following comment was made:"
 auto-comment ]
 ipn-extra-information =
 "The following information types were converted:"
 encoded-info
 ipn-content-return = "The Original Message is not available"
 / "The Original Message follows:"
 preferred-recipient = mailbox
 receipt-time = date-time
 auto-comment = printablestring
 ipn-suppl = printablestring
 discard-reason = "Expired" / "Obsoleted" /
 "User Subscription Terminated" / "IPM Deleted"
 acknowledgement-mode = "Manually" / "Automatically"
 ipms-field = "Supersedes" ":" 1*msg-id
 / "Expires" ":" date-time
 / "Reply-By" ":" date-time
 / "Importance" ":" importance
 / "Sensitivity" ":" sensitivity
 / "Autoforwarded" ":" boolean
 / "Incomplete-Copy" ":"
 / "Content-Language" ":" 1#language
 / "Message-Type" ":" message-type
 / "Discarded-X400-IPMS-Extensions" ":"
 1#object-identifier
 / "Autosubmitted" ":" autosubmitted
 importance = "low" / "normal" / "high"
 sensitivity = "Personal" / "Private" /
 "Company-Confidential"
 language = 2*ALPHA [ "(" language-description ")" ]
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 language-description = printable-string
 message-type = "Delivery Report"
 / "InterPersonal Notification"
 / "Multiple Part"
 autosubmitted = "not-auto-submitted"
 / "auto-generated"
 / "auto-replied"
 / "auto-forwarded"
 redirect-comment = redirect-first *( redirect-subsequent )
 redirect-first = "Originally To:" mailbox "Redirected on"
 date-time "To:" redirection-reason
 redirect-subsequent = mailbox "Redirected Again on"
 date-time "To:" redirection-reason
 redirection-history-item = "intended recipient" mailbox
 "redirected to" redirection-reason
 "on" date-time
 redirection-reason =
 "Recipient Assigned Alternate Recipient"
 / "Originator Requested Alternate Recipient"
 / "Recipient MD Assigned Alternate Recipient"
 / "Directory Look Up"
 / "Alias"
 subject-line = "Delivery-Report" "(" status ")"
 [ "for" destination ]
 status = "success" / "failure" / "success and failures"
 destination = mailbox / "MTA" word
 extended-heading =
 "Prevent-NonDelivery-Report" ":"
 / "Generate-Delivery-Report" ":"
 / "Alternate-Recipient" ":" prohibition
 / "Disclose-Recipients" ":" prohibition
 / "X400-Content-Return" ":" prohibition
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Appendix F - Text format for MCGAM distribution
1. Text Formats
 This appendix defines text formats for exchange of four types of
 mapping.
 1. Domain Name Space -> OR Address Space MCGAM
 2. OR Address Space -> Domain Name Space MCGAM
 3. Domain Name Space -> OR Address of preferred gateway
 4. OR Address Space -> Domain Name of preferred gateway
2. Mechanisms to register and to distribute MCGAMs
 There is a well known set of MCGAM tables.
 The global coordination of the mapping rules is a part of the DANTE
 MailFLOW Project. New mapping rules may be defined by the authority
 responsible for the relevant name space. The rules need to be
 registered with a national mapping registration authority, which in
 turn passes them on to the central mapping registration authority.
 All the collected mapping rules are merged together into the globally
 coordinated mapping tables by the MailFLOW Project Team. The tables
 are available from the national mapping registration authorities.
 To get a contact address of the mapping registration authority for
 the respective country or more information about the MailFLOW Project
 contact:
 SWITCH
 MailFLOW Project Team
 Limmatquai 138
 8001 Zuerich
 Switzerland
 email: mailflow@mailflow.dante.net
 S=MailFLOW;O=MailFLOW;P=DANTE;A=mailnet;C=fi;
 fax: +41 1 268 15 68
 tel: +41 1 268 15 20
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3. Syntax Definitions
 An address syntax is defined, which is compatible with the syntax
 used for 822.domains. By representing the OR addresses as domains,
 all lookups can be mechanically implemented as domain -> domain
 mappings. This syntax defined is initially for use in table format,
 but the syntax is defined in a manner which makes it suitable to be
 adapted for use with the Domain Name Service. This syntax allows
 for a general representation of OR addresses, so that it can be used
 in other applications. Not all attributes are used in the table
 formats defined.
 To allow the mapping where a level of the hierarchy is omitted, the
 pseudo-value "@" (not a printable string character) is used to
 indicate omission of a level in the hierarchy. This is distinct from
 the form including the element with no value, although a correct
 X.400 implementation will interpret both in the same manner.
 This syntax is not intended to be handled by users.
 dmn-or-address = dmn-part *( "." dmn-part )
 mn-part = dmn-attribute "$" value
 dmn-attribute = standard-type
 / "~" dmn-printablestring
 value = dmn-printablestring
 / "@"
 dmn-printablestring =
 = *( dmn-char / dmn-pair )
 dmn-char = <"{", "}", "*", and any ps-char
 except ".">
 dmn-pair = "\."
 An example usage:
 ~ROLE$Big\.Chief.ADMD$ATT.C$US
 PRMD$DEC.ADMD$@.C$US
 The first example illustrates quoting of a "." and a domain define
 attribute (ROLE). The second example illustrates omission of the
 ADMD level. There shall be a strict ordering of all components in
 this table, with the most significant components on the RHS. This
 allows the encoding to be treated as a domain.
 Various further restrictions are placed on the usage of dmn-or-
 address in the address space mapping tables.
 a. Only C, ADMD, PRMD, O, and up to four OUs may be used.
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 b. No components shall be omitted from this hierarchy, although
 the hierarchy may terminate at any level. If the mapping is
 to an omitted component, the "@" syntax is used.
4. Table Lookups
 When determining a match, there are aspects which apply to all
 lookups. Matches are always case independent. The key for all three
 tables is a domain. The longest possible match shall be obtained.
 Suppose the table has two entries with the following keys:
 K.L
 J.K.L
 Domain "A.B.C" will not return any matches. Domain "I.J.K.L"
 will match the entry "J.K.L:.
5. Domain -> OR Address MCGAM format
 The BNF is:
 domain-syntax "#" dmn-or-address "#"
 EBNF.domain-syntax is defined in Section 4.2. Note that the trailing
 "#" is used for clarity, as the dmn-or-address syntax might lead to
 values with trailing blanks. Lines starting with "#" are comments.
 For example:
 AC.UK#PRMD$UK\.AC.ADMD$GOLD 400.C$GB#
 XEROX.COM#O$Xerox.ADMD$ATT.C$US#
 GMD.DE#O$@.PRMD$GMD.ADMD$DBP.C$DE#
 A domain is looked up to determine the top levels of an OR Address.
 Components of the domain which are not matched are used to build the
 remainder of the OR address, as described in Section 4.3.4.
6. OR Address -> Domain MCGAM format
 The syntax of this table is:
 dmn-or-address "#" domain-syntax "#"
 For example:
 #
 # Mapping table
 #
 PRMD$UK\.AC.ADMD$GOLD 400.C$GB#AC.UK#
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 The OR Address is used to generate a domain key. It is important to
 order the components correctly, and to fill in missing components in
 the hierarchy. Use of this mapping is described in Section 4.3.2.
7. Domain -> OR Address of Preferred Gateway table
 This uses the same format as the domain -> OR address MCGAM table.
 In this case, the restriction to only use C/ADMD/PRMD/O/OU does not
 apply. Use of this mapping is described in Section 4.3.4. A domain
 cannot appear in this table and in the domain to OR Address table.
8. OR Addresss -> domain of Preferred Gateway table
 This uses the same format as the OR Address -> domain MCGAM table.
 Use of this mapping is described in Section 4.3.5. An OR Address
 cannot appear in this table and in the OR Address to domain table.
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Appendix G - Conformance
 This appendix defines a number of options, which a conforming gateway
 shall specify. Conformance to this specification shall not be
 claimed if any of the mandatory features are not implemented. A
 specification of conformance may list the service elements of Chapter
 2, in order to be clear that full conformance is provied. In
 particular:
 - Formats for all fields shall be followed.
 - The gateway shall enable MCGAMs to be used.
 - Formats for subject lines, delivery reports and IPNs shall
 be followed. A system which followed the syntax, but
 translated text into a language other than english would be
 conformant.
 - RFC 1137 shall not be followed when mapping to SMTP.
 - All mappings of trace shall be implemented.
 - There shall be a mechanism to access all three global
 mappings.
 - RFC 2157 shall be followed for mapping body parts.
 - When it is specified that a MIME format message is
 generated, RFC 2045 shall be followed.
 A gateway shall specify:
 - Which Interent Message Transport (822-MTS) protocols are
 supported. If SMTP is supported, Appendex A of MIXER shall
 be used.
 - Which X.400 versions are supported (84, 88, 92).
 - Which mechanisms (table, X.500, DNS) are supported to access
 MCGAMs.
 - The mechanism or mechanisms by which the global mapping
 information is accessed.
 The following are optional parts of this specification. A conforming
 implementation shall specify which of these it supports.
 - Support for the extension mappings of Appendix C.
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RFC 2156 MIXER January 1998
 - Support for returning illegal format content in a delivery
 report
 - Which address interpretation heuristics are supported
 (4.3.4.1)
 - If RFC 987 generated message ids are handled in a backwards
 compatible manner (4.7.3.6)
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Appendix H - Change History: RFC 987, 1026, 1138, 1148
 RFC 987 was the original document, and contained the key elements of
 this specification. It was specific to X.400(1984). RFC 1026
 specified a small number of necessary changes to RFC 987.
 RFC 1138 was based on the RFC 987 work. It contained an editorial
 error, and was reissued a few months later as RFC 1148. RFC 1148
 will be referred to here, as it is the document which is widely
 referred to elsewhere. The major goal of RFC 1148 was to upgrade RFC
 987 to X.400(1988). It did this, but did not obsolete RFC 987, which
 was recommended for use with X.400(1984). This appendix summarises
 the changes made in going from RFC 987 to RFC 1148.
 RFC 1148 noted the following about its upgrade from RFC 987:
 Unnecessary change is usually a bad idea. Changes on the RFC 822
 side are avoided as far as possible, so that RFC 822 users do not
 see arbitrary differences between systems conforming to this
 specification, and those following RFC 987. Changes on the X.400
 side are minimised, but are more acceptable, due to the mapping onto
 a new set of services and protocols.
1. Introduction
 The model has shifted from a protocol based mapping to a service
 based mapping. This has increased the generality of the
 specification, and improved the model. This change affects the
 entire document.
 A restriction on scope has been added.
2. Service Elements
 - The new service elements of X.400 are dealt with.
 - A clear distinction is made between origination and
 reception
3. Basic Mappings
 - Add teletex support
 - Add object identifier support
 - Add labelled integer support
 - Make PrintableString <-> ASCII mapping reversible
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 - The printable string mapping is aligned to the NBS mapping
 derived from RFC 987.
4. Addressing
 - Support for new addressing attributes
 - The message ID mapping is changed to not be table driven
5. Detailed Mappings
 - Define extended IPM Header, and use instead of second body
 part for RFC 822 extensions
 - Realignment of element names
 - New syntax for reports, simplifying the header and
 introducing a mandatory body format (the RFC 987 header
 format was unusable)
 - Drop complex autoforwarded mapping
 - Add full mapping for IP Notifications, defining a body
 format
 - Adopt an MTS Identifier syntax in line with the OR Address
 syntax
 - A new format for X400 Trace representation on the RFC 822
 side
6. Appendices
 - Move Appendix on restricted 822 mappings to a separate RFC
 - Delete Phonenet and SMTP Appendixes
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Appendix I - Change History: RFC 1148 to RFC 1327
1. General
 - The scope of the document was changed to cover X.400(1984),
 and so obsolete RFC 987.
 - Changes were made to allow usage to connect RFC 822 networks
 using X.400
 - Text was tightened to be clear about optional and mandatory
 aspects
 - A good deal of clarification
 - A number of minor EBNF errors
 - Better examples are given
 - Further X.400 upper bounds are handled correctly
2. Basic Mappings
 - The encoding of object identifier is changed slightly
3. Addressing
 - A global mapping of domain to preferred gateway is
 introduced.
 - An overflow mechanism is defined for RFC 822 addresses of
 greater than 128 bytes
 - Changes were made to improve compatibility with the PDAM on
 writing OR Addresses.
 + The PD and Terminal Type keywords were aligned to the
 PDAM. It is believed that minimal use has been made of
 the RFC 1148 keywords.
 + P and A are allowed as alternate keys for PRMD and ADMD
 + Where keywords are different, the PDAM keywords are
 alternatives on input. This is mandatory.
4. Detailed Mappings
 - The format of the Subject: lines is defined.
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RFC 2156 MIXER January 1998
 - Illegal use (repetition) of the heading EXTENSION is
 corrected, and a new object identifier assigned.
 - The Delivery Report format is extensively revised in light
 of operational experience.
 - The handling of redirects is significantly changed, as the
 previous mechanism did not work.
5. Appendices
 - An SMTP appendix is added, allowing optional use of the VRFY
 command to improve probe information.
 - Handling of JNT Mail Acknowledge-To is changed slightly.
 - A DDA JNT-MAIL is allowed on input.
 - The format definitions of Appendix F are explained further,
 and a third table definition added.
 - An appendix on use with X.400(1984) is added.
 - Optional extensions are defined to give RFC 822 access to
 further X.400 facilities.
 - An appendix on conformance is added.
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Appendix J - Change History: RFC 1327 to this Document
1. General
 This update is primarily for stability, and to fold in compatibility
 for MIME and to add support for the new NOTARY delivery status
 notifications. Other general changes:
 - Various editorial updates
 - Minor EBNF errors
 - Reference to mapping table support by DNS and X.500.
 - Alignment to X.400(92)
 - Assignment of a new object identifier
 - Removal of specification relating to body mapping, which is
 now defined in RFC 2157.
2. Service Elements
 - Support of Auto-Submitted service
3. Basic Mappings
 - Comments shall not be used in new headers, to remove parsing
 ambiguity
 - RFC 1522 encoding may be used as an alternative to X.408
 downgrade, where appropriate.
 - Correct handling of RFC 822 four year dates.
4. Addressing
 - Replaced the mandatory global address mapping with MCGAMs.
 - Add codes and add a heuristic to align to the standard X.400
 form of writing OR Addresses.
 - Improved text on ordering heuristic
 - Leading "/" interpretation added
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 - All bar one of the address mapping heuristics made
 mandatory.
 - Interpretation of domain defined attribute "RFC-822" made
 mandatory in all cases
 - Make report request comments optional
5. Detailed Mappings
 - Comments no longer maps to separate body part
 - Allow Languages to be multi-valued
 - Change Content-Identifier to X400-Content-Identifier, in
 order to avoid confusion with MIME.
 - Reverse mapping of MIXER defined fields made mandatory
 - "Expiry-Date:" changed to "Expires:".
 - "Obsoletes:" changed to "Supersedes:".
 - Define correct handling when "Resent-Date:" is present.
6. Appendices
 - Change "Content-Return" to "X400-Content-Return" in Appendix
 C.
 - Relaxation of restrictions on mapping 3 in Appendix F.
 - Add linkage to HARPOON in Appendix B.
 - RFC 2157 added to the conformance statement of Appendix
 G.
 - Added Appendix L, with ASN. Summary.
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Appendix L - ASN.1 Summary
 MIXER Definitions { iso org(3) dod(6) internet(1) mail(7)
 mixer(1) mixer-core(3) definitions(1) }
 DEFINITIONS IMPLICIT TAGS ::=
 BEGIN
 -- exports everything
 IMPORTS
 EXTENSION FROM
 MTSAbstractService {join-iso-ccit mhs-motis(6) mts(3)
 modules(0) mts-abstract-service(1) }
 HEADING-EXTENSION FROM
 IPMSAbstractService {join-iso-ccit mhs-motis(6) ipms(1)
 modules(0) abstract-service(3) }
 rfc-822-field HEADING-EXTENSION
 VALUE RFC822FieldList
 ::= id-rfc-822-field-list
 RFC822FieldList ::= SEQUENCE OF RFC822Field
 RFC822Field ::= IA5String
 dsn-header-list EXTENSION
 RFC822FieldList
 ::= id-dsn-header-list
 dsn-field-list EXTENSION
 RFC822FieldList
 ::= id-dsn-field-list
 internet ::= OBJECT IDENTIFIER { iso org(3) dod(6) 1 } -- from RFC
 1155
 mail OBJECT IDENTIFIER ::= { internet 7 } -- IANA assigned
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 mixer OBJECT IDENTIFIER ::= { mail mixer(1) } -- inherited from RFC
 1495
 mixer-core OBJECT IDENTIFIER ::= { mixer core(3) }
 id-rfc-822-field-list OBJECT IDENTIFIER ::= {mixer-core 2}
 id-dsn-header-list OBJECT IDENTIFIER ::= {mixer-core 3}
 id-dsn-field-list OBJECT IDENTIFIER ::= {mixer-core 4}
 eit-mixer OBJECT IDENTIFIER ::= {mixer-core 5}
 -- the MIXER pseudo-EIT
 END -- MIXER ASN.1
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SECURITY CONSIDERATIONS
 Security issues are not discussed in this memo.
AUTHOR'S ADDRESS
 Steve Kille
 Isode Ltd
 The Dome
 The Square
 Richmond
 TW9 1DT
 England
 Phone: +44-181-332-9091
 Internet EMail: S.Kille@ISODE.COM
 X.400 Email: I=S; S=Kille; P=Isode; A=Mailnet; C=FI;
 UFN: S.Kille, Isode, GB
References
 1. CCITT , "Recommendations X.400", Message Handling Systems:
 System Model - Service Elements, October 1984.
 2. Allocchio, C., "MaXIM11 - Mapping between X.400 / Internet
 Mail and Mail-11 mail", RFC 2162, January 1998.
 3. Allocchio, C., "Using the Internet DNS to Distribute MIXER
 Conformant Global Address Mapping (MCGAM)", RFC 2163,
 January 1998.
 4. Alvestrand, H., Kille, S., Miles, R., Rose, M., and S.
 Thompson, "Mapping between X.400 and RFC-822 Message
 Bodies", RFC 1495, August 1993.
 5. Alvestrand, H., Romaguera, J., and K. Jordan, "Rules for
 Downgrading Messages for X.400(88) to X.400(84) When MIME
 Content-Types are Present in the Messages (Harpoon)", RFC
 1496, August 1993.
 6. Alvestrand, H., and S. Thompson, "Equivalences between X.400
 and RFC-822 Message Bodies", RFC 1494, August 1993.
 7. Alvestrand, H., "Tags for the Identification of Languages",
 RFC 1766, March 1995.
Kille Standards Track [Page 141]
RFC 2156 MIXER January 1998
 8. Alvestrand, H., "Mapping between X.400 and RFC-822/MIME
 Message Bodies", RFC 2157, January 1998.
 9. Freed, N., and N. Borenstein, "Multipurpose Internet
 Mail Extensions (MIME) Part One: Format of Internet Message
 Bodies", RFC 2045, November 1996.
 10. Braden, R., "Requirements for Internet Hosts -- Application
 and Support", STD 3, RFC 1123, October 1989.
 11. CCITT/ISO, "CCITT Recommendations X.420/ ISO/IEC 10021-7,"
 Message Handling Systems: Interpersonal Messaging System,
 Dec 1988.
 12. CCITT/ISO, "CCITT Recommendations X.411/ ISO/IEC 10021-4,"
 Message Handling Systems: Message Transfer System: Abstract
 Service Definition and Procedures, Dec 1988.
 13. CCITT/ISO, "CCITT Recommendations X.400/ ISO/IEC 10021-1,"
 Message Handling: System and Service Overview , Dec 1988.
 14. CCITT/ISO, "Specification of Abstract Syntax Notation One
 (ASN.1)," CCITT Recommendation X.208 / ISO/IEC 8824, Dec
 1988.
 15. CCITT/ISO, "CCITT Recommendations X.400/ ISO/IEC 10021-1,"
 Message Handling: System and Service Overview , Dec 1992.
 16. Crocker, D., "Standard of the Format of ARPA Internet Text
 Messages", STD 11, RFC 822, August 1982.
 17. Kille, S., "Mapping Between X.400 and RFC 822", UK Academic
 Community Report (MG.19) / RFC 987, June 1986.
 18. Kille, S., "Addendum to RFC 987", UK Academic Community
 Report (MG.23) / RFC 1026, August 1987.
 19. Kille, S., "Mapping Between X.400(1988) / ISO 10021 and RFC
 822", RFC 1138, October 1989.
 20. Kille, S., "Mapping Between X.400(1988) / ISO 10021 and RFC
 822", RFC 1148, March 1990.
 21. Kille, S., "Mapping Between X.400(1988) / ISO 10021 and RFC
 822", RFC 1327, May 1992.
 22. Kille, S., "X.400 1988 to 1984 downgrading", RFC 1328, May
 1992.
Kille Standards Track [Page 142]
RFC 2156 MIXER January 1998
 23. Kille, S., "A String Encoding of Presentation Address", RFC
 1278, November 1992.
 24. Kille, S., "A String Representation of Distinguished Name",
 RFC 1485, January 1992.
 25. Kille, S., "Using the OSI Directory to achieve User
 Friendly Naming", RFC 1484, January 1992.
 26. Kille, S., "Use of an X.500/LDAP directory to support MIXER
 address mapping", RFC 2164, January 1998.
 27. Koorland, N., "Message Attachmment Work Group (MAWG): MAWG
 Feasibility Project Guide," EMA Report, Version 1.5, Nov
 1995.
 28. Moore, K., and G. Vaudreuil, "An Extensible Message Format for
 Delivery Status Notifications", RFC 1894, January 1996.
 29. Moore, K., "SMTP Service Extensions for Delivery Status
 Notifications", RFC 1891, Januaty 1996.
 30. Postel, J., "SIMPLE MAIL TRANSFER PROTOCOL", STD 10, RFC 821,
 August 1982.
Kille Standards Track [Page 143]
RFC 2156 MIXER January 1998
Full Copyright Statement
 Copyright (C) The Internet Society (1998). All Rights Reserved.
 This document and translations of it may be copied and furnished to
 others, and derivative works that comment on or otherwise explain it
 or assist in its implementation may be prepared, copied, published
 and distributed, in whole or in part, without restriction of any
 kind, provided that the above copyright notice and this paragraph are
 included on all such copies and derivative works. However, this
 document itself may not be modified in any way, such as by removing
 the copyright notice or references to the Internet Society or other
 Internet organizations, except as needed for the purpose of
 developing Internet standards in which case the procedures for
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 followed, or as required to translate it into languages other than
 English.
 The limited permissions granted above are perpetual and will not be
 revoked by the Internet Society or its successors or assigns.
 This document and the information contained herein is provided on an
 "AS IS" basis and THE INTERNET SOCIETY AND THE INTERNET ENGINEERING
 TASK FORCE DISCLAIMS ALL WARRANTIES, EXPRESS OR IMPLIED, INCLUDING
 BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE INFORMATION
 HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED WARRANTIES OF
 MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.
Kille Standards Track [Page 144]