Interactive Mail Access Protocol: Version 3
RFC 1203

Network Working Group J. Rice
Request for Comments: 1203 Stanford
Obsoletes: RFC 1064 February 1991
 INTERACTIVE MAIL ACCESS PROTOCOL - VERSION 3
Status of this Memo
 This RFC suggests a method for workstations to access mail
 dynamically from a mailbox server ("repository"). This RFC specifies
 a standard for the SUMEX-AIM community and an Experimental Protocol
 for the Internet community. Discussion and suggestions for
 improvement are requested. Please refer to the current edition of
 the "IAB Official Protocol Standards" for the standardization state
 and status of this protocol. Distribution of this memo is unlimited.
Scope
 The following document is a modified version of RFC 1064, the
 definition of the IMAP2 protocol. This RFC has been written
 specifically as a counter proposal to RFC 1176, which itself proposes
 modifications to IMAP2. Sadly, RFC 1176 was made without internal
 consultation with the IMAP community, so we are in a position of
 feeling we have to present a counter proposal to what, if we do not
 act, will become a de facto standard. The reasons for this counter
 proposal are numerous but fall mostly into the following categories:
 - IMAP2 is insufficiently powerful for a number of server/client
 interactions which we believe to be important. RFC 1176
 negligibly enhances the functionality of IMAP2.
 - IMAP2 makes what we believe to be an erroneous definition for
 unsolicited vs. solicited data. IMAP3 as specified herein
 attempts to correct this. RFC 1176 makes no effort to remedy
 these problems.
 - RFC 1176 has explicitly modified the intent of RFC 1064 by
 allowing the server to make assumptions about the client's
 caching architecture. We believe this to be a grave error
 and do not support it in this proposal.
 - RFC 1176 specifies a number of "optional" features in the
 protocol without specifying a suitable metaprotocol by which
 servers and clients can adequately negotiate over the set of
 implemented features. This proposal specifies a mechanism
 by which servers and clients can come to an unambiguous
 understanding about which features are usable by each party.
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RFC 1203 IMAP3 February 1991
 - RFC 1176 pays only lip-service to being network protocol
 independent and, in fact assumes the use of TCP/IP. Neither
 RFC 1064 nor this proposal make any such assumption.
 Although there are numerous other detailed objections to RFC 1176, we
 believe that the above will serve to show that we believe strongly in
 the importance of mailbox abstraction level mail protocols and, after
 a couple of years of use of IMAP2 under RFC 1064 we believe that we
 have a good enough understanding of the issues involved to be able to
 take the next step.
 It is important to take this next step because of the rapid pace of
 both mail system and user interface development. We believe that,
 for IMAP not to die in its infancy, IMAP must be ready to respond to
 emerging ISO and RFC standards in mail, such as for multi-media mail.
 We believe that RFC 1176 not only provides a very small increment in
 functionality over RFC 1064 but also adds a number of bugs, which
 would be detrimental to the IMAP cause. Thus we propose the
 following definition for IMAP3.
Compatibility notes:
 In revising the IMAP2 protocol it has been our intent, wherever
 possible to make upwards compatible changes to produce IMAP3. There
 were, however, some places that had to be changed incompatibly in
 order to compensate for either ambiguities in the IMAP2 protocol as
 defined by RFC 1064 or behavior that proved undesirable in the light
 of experience.
 It is our goal, however, that existing IMAP2 clients should still be
 supported and that, at least for the foreseeable future, all IMAP3
 servers will support IMAP2 behavior as their default mode.
 The following are the major differences between this proposal, RFC
 1176 and RFC 1064:
 - In this proposal we specify a difference between "solicited" and
 "unsolicited" data sent from the server. It is generally the
 case that data sent by the server can be sent either in response
 to an explicit request by the client or by the server of its own
 volition. Any data that the server is required to sent to the
 client as the result of a request is said to be solicited and
 carries the same tag as the request that provoked it. Any data
 sent by the server to the client that is not required by the
 protocol is said to be unsolicited and carries the special "*"
 tag. RFC 1176 preserves the original RFC 1064 terminology that
 calls all such data sent by the server "unsolicited" even when
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RFC 1203 IMAP3 February 1991
 it is, in fact, solicited.
 - This proposal introduces the experimental concept of
 distinguishing between Generic, Canonical and Concrete keys,
 allowing the mailbox to be viewed as a relational database
 indexed by these keys. This should allow the IMAP protocol
 to evolve away from its current reliance on RFC 822. RFC 1176
 does not have such a unifying model.
 - The SEARCH command has been changed so as to allow multiple
 simultaneous searches to be made and to allow unsolicited
 search messages to be sent by the server. Such a change is
 essential to allow more sophisticated servers that can process
 commands asynchronously, possibly substantially delaying
 searches over slow backing storage media, for example. It is
 also important to allow servers to be able to send unsolicited
 search messages that might inform the client of interesting
 patterns of messages, such as new and unseen mail.
 - This proposal introduces a specific protocol for the negotiation
 of protocol versions and server features. This is important
 because it allows client/server pairs to come to an agreement on
 what behavior is really available to it. RFC 1176 introduces a
 number of "optional" commands, which are in some way analogous
 to "feature-introduced" commands in this proposal. The principle
 distinction between these is that in RFC 1176 there is no way
 for a client to discover the set of optional commands, nor is
 there a way for it to determine whether a specific command
 really is supported, since RFC 1176 requires the use of the
 "BAD" response if a feature is not supported. There is,
 therefore, no way for the client to determine why the attempted
 command did not work. This also means that, for example, a
 client cannot disable certain user commands or make them
 invisible on menus if they are not supported, since there
 is no way for the client to discover whether the commands are
 indeed supported without trying to execute such a command.
 - This proposal introduces a mechanism for clients to create and
 delete user flags (keywords). This is nor supported in either
 RFC 1176 or RFC 1064, requiring the user to add keys manually
 on the server, generally by editing some form of "init" file.
 - RFC 1064 has no mechanism for determining whether a mailbox is
 readonly or not. RFC 1176 introduces a non-enforced convention
 of encoding data about the readonly status of a mailbox in the
 SELECT message's OK respose comment field. This is not regular
 with respect to the rest of the protocol, in which the comment
 field is used for no purpose other than documentation. This
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RFC 1203 IMAP3 February 1991
 proposal introduces specific protocol additions for the dynamic
 determination and modification of the readonly/readwrite status
 of mailboxes.
Introduction
 The intent of the Interactive Mail Access Protocol, Version 3 (IMAP3)
 is to allow a (possibly unreliable) workstation or similar machine to
 access electronic mail from a reliable mailbox server in an efficient
 manner.
 Although different in many ways from POP2 (RFC 937), IMAP3 may be
 thought of as a functional superset of POP2, and the POP2 RFC was
 used as a model for this RFC. There was a cognizant reason for this;
 RFC 937 deals with an identical problem and it was desirable to offer
 a basis for comparison.
 Like POP2, IMAP3 specifies a means of accessing stored mail and not
 of posting mail; this function is handled by a mail transfer protocol
 such as SMTP (RFC 821). A comparison with the DMSP protocol of
 PCMAIL can be found at the end of "System Model and Philosophy"
 section.
 This protocol assumes a reliable data stream such as provided by TCP
 or any similar protocol. When TCP is used, the IMAP server listens
 on port 220. When CHAOS is used the IMAP server listens for the
 logical contact name "IMAP3".
 Communication in IMAP is defined to be using the ASCII character
 interpretation of data. Communication using other conventions may be
 possible by the selection of features on some servers.
System Model and Philosophy
 Electronic mail is a primary means of communication for the widely
 spread SUMEX-AIM community. The advent of distributed workstations
 is forcing a significant rethinking of the mechanisms employed to
 manage such mail. With mainframes, each user tends to receive and
 process mail at the computer he used most of the time, his "primary
 host". The first inclination of many users when an independent
 workstation is placed in front of them is to begin receiving mail at
 the workstation, and, in fact, many vendors have implemented
 facilities to do this. However, this approach has several
 disadvantages:
 (1) Workstations (especially Lisp workstations) have a software
 design that gives full control of all aspects of the system
 to the user at the console. As a result, background tasks,
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RFC 1203 IMAP3 February 1991
 like receiving mail, could well be kept from running for
 long periods of time either because the user is asking to
 use all of the machine's resources, or because, in the course
 of working, the user has (perhaps accidentally) manipulated
 the environment in such a way as to prevent mail reception.
 This could lead to repeated failed delivery attempts by
 outside agents.
 (2) The hardware failure of a single workstation could keep its
 user "off the air" for a considerable time, since repair of
 individual workstation units might be delayed. Given the
 growing number of workstations spread throughout office
 environments, quick repair would not be assured, whereas a
 centralized mainframe is generally repaired very soon after
 failure.
 (3) It is more difficult to keep track of mailing addresses when
 each person is associated with a distinct machine. Consider
 the difficulty in keeping track of a large number of postal
 addresses or phone numbers, particularly if there was no
 single address or phone number for an organization through
 which you could reach any person in that organization.
 Traditionally, electronic mail on the ARPANET involved
 remembering a name and one of several "hosts" (machines)
 whose name reflected the organization in which the
 individual worked. This was suitable at a time when most
 organizations had only one central host. It is less
 satisfactory today unless the concept of a host is changed
 to refer to an organizational entity and not a particular
 machine.
 (4) It is very difficult to keep a multitude of heterogeneous
 workstations working properly with complex mailing protocols,
 making it difficult to move forward as progress is made in
 electronic communication and as new standards emerge. Each
 system has to worry about receiving incoming mail, routing
 and delivering outgoing mail, formatting, storing, and
 providing for the stability of mailboxes over a variety of
 possible filing and mailing protocols.
 Consequently, while the workstation may be viewed as an Internet host
 in the sense that it implements IP, it should not be viewed as the
 entity which contains the user's mailbox. Rather, a mail server
 machine (sometimes called a "repository") should hold the mailbox,
 and the workstation (hereafter referred to as a "client") should
 access the mailbox via mail transactions. Because the mail server
 machine would be isolated from direct user manipulation, it could
 achieve high software reliability easily, and, as a shared resource,
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RFC 1203 IMAP3 February 1991
 it could achieve high hardware reliability, perhaps through
 redundancy. The mail server could be used from arbitrary locations,
 allowing users to read mail across campus, town, or country using
 more and more commonly available clients. Furthermore, the same user
 may access his mailbox from different clients at different times, and
 multiple users may access the same mailbox simultaneously.
 The mail server acts an an interface among users, data storage, and
 other mailers. The mail access protocol is used to retrieve
 messages, access and change properties of messages, and manage
 mailboxes. This differs from some approaches (e.g., Unix mail via
 NFS) in that the mail access protocol is used for all message
 manipulations, isolating the user and the client from all knowledge
 of how the data storage is used. This means that the mail server can
 utilize the data storage in whatever way is most efficient to
 organize the mail in that particular environment, without having to
 worry about storage representation compatibility across different
 machines.
 In defining a mail access protocol, it is important to keep in mind
 that the client and server form a macrosystem, in which it should be
 possible to exploit the strong points of both while compensating for
 each other's weaknesses. Furthermore, it's desirable to allow for a
 growth path beyond the hoary text-only RFC 822 protocol. Unlike
 POP2, IMAP3 has extensive features for remote searching and parsing
 of messages on the server. For example, a free text search
 (optionally in conjunction with other searching) can be made
 throughout the entire mailbox by the server and the results made
 available to the client without the client having to transfer the
 entire mailbox and searching itself. Since remote parsing of a
 message into a structured (and standard format) "envelope" is
 available, a client can display envelope information and implement
 commands such as REPLY without having any understanding of how to
 parse RFC 822, etc., headers.
 Additionally, IMAP3 offers several facilities for managing a mailbox
 beyond the simple "delete message" functionality of POP2.
 In spite of this, IMAP3 is a relatively simple protocol. Although
 servers should implement the full set of IMAP3 functions, a simple
 client can be written which uses IMAP3 in much the way as a POP2
 client.
 IMAP3 differs from the DMSP protocol of PCMAIL (RFC 1056) in a more
 fundamental manner, reflecting the differing architectures of IMAP
 and PCMAIL. PCMAIL is either an online ("interactive mode"), or
 offline ("batch mode") system. IMAP is primarily an online system in
 which real-time and simultaneous mail access were considered
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RFC 1203 IMAP3 February 1991
 important.
 In PCMAIL, there is a long-term client/server relationship in which
 some mailbox state is preserved on the client. There is a
 registration of clients used by a particular user, and the client
 keeps a set of "descriptors" for each message which summarize the
 message. The server and client synchronize their states when the
 DMSP connection starts up, and, if a client has not accessed the
 server for a while, the client does a complete reset (reload) of its
 state from the server.
 In IMAP, the client/server relationship lasts only for the duration
 of the IMAP3 connection. All mailbox state is maintained on the
 server. There is no registration of clients. The function of a
 descriptor is handled by a structured representation of the message
 "envelope". This structure makes it unnecessary for a client to know
 anything about RFC 822 parsing. There is no synchronization since
 the client does not remember state between IMAP3 connections. This
 is not a problem since in general the client never needs the entire
 state of the mailbox in a single session, therefore there isn't much
 overhead in fetching the state information that is needed as it is
 needed.
 There are also some functional differences between IMAP3 and DMSP.
 DMSP has functions for sending messages, printing messages, and
 changing passwords, all of which are done outside of IMAP3. DMSP has
 16 binary flags of which 8 are defined by the system. IMAP has flag
 names; there are currently 5 defined system flag names and a facility
 for some number (29 in the current implementations) of user flag
 names. IMAP3 has a sophisticated message search facility in the
 server to identify interesting messages based on dates, addresses,
 flag status, or textual contents without compelling the client to
 fetch this data for every message.
 It was felt that maintaining state on the client is advantageous only
 in those cases where the client is only used by a single user, or if
 there is some means on the client to restrict access to another
 user's data. It can be a serious disadvantage in an environment in
 which multiple users routinely use the same client, the same user
 routinely uses different clients, and where there are no access
 restrictions on the client. It was also observed that most user mail
 access is to a relatively small set of "interesting" messages, which
 were either "new" mail or mail based upon some user-selected
 criteria. Consequently, IMAP3 was designed to easily identify those
 "interesting" messages so that the client could fetch the state of
 those messages and not those that were not "interesting".
 One crucial philosophical difference between IMAP and other common
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RFC 1203 IMAP3 February 1991
 mail protocols is that IMAP is a mailbox access protocol, not a
 protocol for manipulating mail files. In the IMAP model, unlike
 other mail system models in which mail is stored in a linear mail
 file, no specification is made for the implementation architecture
 for mail storage. Servers may choose to implement mailboxes as files
 but this is a detail of which the client can be totally unaware.
 What is more, in the IMAP model, mailboxes are viewed as mappings
 from keys into values. There are broadly three types of keys,
 generic, canonical and concrete. Generic keys are generic, mail
 protocol independent keys defined by IMAP which are meaningful across
 multiple mail encoding formats. An example of such a generic key
 might be "TO", which would be associated with the "To:" field of an
 RFC 822 format message.
 Canonical keys represent the way in which the server can associate
 values that are generally "about" a certain key concept, possibly
 integrating several mail format specific fields, without having to
 worry the client with the particular details of any particular
 message format. Thus, the canonical TO key (called $TO) could denote
 anything that could reasonably be construed as being directed towards
 someone. Hence, in an RFC 822 message the server could find the
 union of the "To:", "Resent-To", "Apparently-To:" and "CC:" fields to
 be the appropriate value associated with the canonical $TO key.
 Concrete keys allow the client to gain access to certain mail format
 specific concepts, that are not pre-specified by the IMAP protocol,
 in a well defined manner. For example, If the client asks for the
 value associated with the "APPARENTLY-TO" key then, if the message
 were to be in RFC 822 format, the server would look for a header
 field called "Apparently-To:". If no such field is found or the
 field is not implemented or meaningful for the particular message
 format then the server will respond with the null value, called NIL,
 indicating the non-existence of the field.
 Thus, IMAP servers are at liberty to implement mailboxes as a
 relational databases if it seems convenient. Indeed, we anticipate
 that future mail systems will tend to use database technology for the
 storage and indexing of mailboxes as a result of the pressure caused
 by the increasing size of mailboxes.
 Although for historical reasons IMAP is currently somewhat closely
 associated with RFC 822, we anticipate that future developments in
 IMAP will remove these mail format specific components and will move
 towards the generic model mentioned above. This will allow IMAP more
 easily to incorporate such things as multi-media mail.
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RFC 1203 IMAP3 February 1991
The Protocol
 The IMAP3 protocol consists of a sequence of client commands and
 server responses to those commands, with extra information from the
 server data being sent asynchronously to and independent to the
 responses to client commands. Unlike most Internet protocols,
 commands and responses are tagged. That is, a command begins with a
 unique identifier (typically a short alphanumeric sequence such as a
 Lisp "gensym" function would generate e.g., A0001, A0002, etc.),
 called a tag. The response to this command is given the same tag
 from the server.
 We distinguish between data sent by the server as the result of a
 client request, which we term "SOLICITED" and data sent by the server
 not as the result of a client request, which we term "UNSOLICITED".
 The server may send unsolicited data at any time that would not
 fragment another piece of data on the same stream rendering it
 unintelligible. The server is contractually required, however, to
 return all data that is solicited by the client before the return of
 the completion signal for that command, i.e., all solicited data must
 be returned within the temporal extent of the request/completion
 acknowledgement wrapper. This does not, however, preclude the
 simultaneous processing of multiple requests by the client, it simply
 requires that the client be confident that it has all the requested
 data when a request finishes. This allows the implementation of both
 synchronous and asynchronous clients.
 Solicited data is identified by the tag of the initial request by the
 client. Unsolicited data is identified by the special reserved tag
 of "*". There is another special reserved tag, "+", discussed below.
 Note: the tagging of SOLICITED data is only permitted for a selected
 server version other than 2.0.
 No assumptions concerning serial or monolithic processing by the
 server can be made by a correct client. The server is at liberty to
 process multiple requests by the same client in any order. This
 allows servers to process costly searches over mailboxes on slow
 backing storage media in the background, while still preserving
 interactive performance. Clients can, however, assume the
 serialization of the request/data/completion behavior mentioned
 above.
 When a connection is opened the server sends an unsolicited OK
 response as a greeting message and then waits for commands. When
 commands are received the server acts on them and responds with
 responses, often interspersed with data.
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RFC 1203 IMAP3 February 1991
 The client opens a connection, waits for the greeting, then sends a
 LOGIN command with user name and password arguments to establish
 authorization. Following an OK response from the server, the client
 then sends a SELECT command to access the desired mailbox. The
 user's default mailbox has a special reserved name of "INBOX" which
 is independent of the operating system that the server is implemented
 on. The server will generally send a list of valid flags, number of
 messages, and number of messages arrived since last access for this
 mailbox as solicited data, followed by an OK response. The client
 may terminate access to this mailbox and access a different one with
 another SELECT command.
 Because the SELECT command affects the state of the server in a
 fundamental way, the server is required to process all outstanding
 commands for any given mailbox before sending the OK tag for the
 SELECT command. Thus, the client will always know that all responses
 before an OK SELECT response will refer to the old mailbox and all
 responses following it will apply to the new mailbox.
 Because, in the real world, local needs or experimental work will
 dictate that servers will support both supersets of the defined
 behavior and incompatible changes, servers will support a
 SELECT.VERSION command and a SELECT.FEATURES command, the purpose of
 which is to allow clients to select the overall behavior and specific
 features that they want from a server. The default behavior of any
 server is to process commands and to have interaction syntax the same
 as is specified by IMAP2 in RFC 1064. A server may not behave in any
 other manner unless the SELECT.VERSION or SELECT.FEATURES commands
 are used to select different behavior.
 Over time, when groups of generally useful changes to the current,
 default behavior of the server are found, these will be collected
 together and incorporated in such a way that all of the features can
 be selected simply by selecting a particular major version number of
 the protocol. It should be noted that the version numbers (both
 major and minor) selected by the SELECT.VERSION command denote
 versions of the IMAP protocol, not versions of the server per se.
 Thus, although in general changes to the protocol specification will
 be made in such a way that they are upwards compatible, this cannot
 be guaranteed. No client should rely on tests of the form "if
 major_version > 2 then..." being valid for all protocol versions,
 since incompatible changes might be made in the future.
 The client reads mailbox information by means of FETCH commands. The
 actual data is transmitted via the solicited data mechanism (that is,
 FETCH should be viewed as poking the server to include the desired
 data along with any other data it wishes to transmit to the client).
 There are three major categories of data which may be fetched.
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 The first category is that data which is associated with a message as
 an entity in the mailbox. There are presently three such items of
 data: the "internal date", the "RFC 822 size", and the "flags". The
 internal date is the date and time that the message was placed in the
 mailbox. The RFC 822 size is subject to deletion in the future; it
 is the size in bytes of the message, expressed as an RFC 822 text
 string. Current clients only use it as part of a status display
 line. The flags are a list of status flags associated with the
 message (see below). All of the first category data can be fetched
 by using the macro-fetch word "FAST"; that is, "FAST" expands to
 "(FLAGS INTERNALDATE RFC822.SIZE)".
 The second category is that data which describes the composition and
 delivery information of a message; that is, information such as the
 message sender, recipient lists, message-ID, subject, etc. This is
 the information which is stored in the message header in RFC 822
 format message and is traditionally called the "envelope". [Note:
 this should not be confused with the SMTP (RFC 821) envelope, which
 is strictly limited to delivery information.] IMAP3 defines a
 structured and unambiguous representation for the envelope which is
 particularly nice for Lisp-based parsers. A client can use the
 envelope for operations such as replying and not worry about RFC 822
 at all. Envelopes are discussed in more detail below. The first and
 second category data can be fetched together by using the macro-fetch
 word "ALL"; that is, "ALL" expands to "(FLAGS INTERNALDATE
 RFC822.SIZE ENVELOPE)".
 The third category is that data which is intended for direct human
 viewing. The present RFC 822 based IMAP3 defines three such items:
 RFC822.HEADER, RFC822.TEXT, and RFC822 (the latter being the two
 former appended together in a single text string). Fetching "RFC822"
 is equivalent to typing the RFC 822 representation of the message as
 stored on the mailbox without any filtering or processing.
 Typically, a client will "FETCH ALL" for some or all of the messages
 in the mailbox for use as a presentation menu, and when the user
 wishes to read a particular message will "FETCH RFC822.TEXT" to get
 the message body. A more primitive client could, of course, simply
 "FETCH RFC822" a la POP2-type functionality.
 The client can alter certain data by means of a STORE command. As an
 example, a message is deleted from a mailbox by a STORE command which
 includes the \DELETED flag as one of the flags being set.
 Other client operations include copying a message to another mailbox
 (COPY command), permanently removing deleted messages (EXPUNGE
 command), checking for new messages (CHECK command), and searching
 for messages which match certain criteria (SEARCH command).
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 The client terminates the session with the LOGOUT command. The
 server returns a "BYE" followed by an "OK".
A Typical Scenario
 Client Server
 ------ ------
 {Wait for Connection}
 {Open Connection} -->
 <-- * OK IMAP3 Server Ready
 {Wait for command}
 A001 SUPPORTED.VERSIONS -->
 <-- * SUPPORTED.VERSIONS ((2 0 )
 (3 0 EIGHT.BIT.TRANSPARENT
 AUTO.SET.SEEN
 TAGGED.SOLICITED))
 A001 OK Supported Versions returned.
 {Wait for command}
 A002 SELECT.VERSION (3 0) -->
 <-- A002 OK Version 3.0 Selected.
 {Wait for command}
 A002 SELECT.FEATURES TAGGED.SOLICITED -->
 <-- A002 OK Features selected.
 {Wait for command}
 A003 LOGIN Fred Secret -->
 <-- A003 OK User Fred logged in
 {Wait for command}
 A004 SELECT INBOX -->
 <-- A004 FLAGS (Meeting Notice \Answered
 \Flagged \Deleted \Seen)
 <-- A004 19 EXISTS
 <-- A004 2 RECENT
 <-- A004 OK Select complete
 {Wait for command}
 A005 FETCH 1:19 ALL -->
 <-- A005 1 Fetch (......)
 ...
 <-- A005 18 Fetch (......)
 <-- A005 19 Fetch (......)
 <-- A005 OK Fetch complete
 {Wait for command}
 A006 FETCH 8 RFC822.TEXT -->
 <-- A006 8 Fetch (RFC822.TEXT {893}
 ...893 characters of text...
 <-- )
 <-- A006 OK Fetch complete
 {Wait for command}
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RFC 1203 IMAP3 February 1991
 A007 STORE 8 +Flags \Deleted -->
 <-- A007 8 Store (Flags (\Deleted
 \Seen))
 <-- A007 OK Store complete
 {Wait for command}
 A008 EXPUNGE -->
 <-- A008 19 EXISTS
 <-- A008 8 EXPUNGE
 <-- A008 18 EXISTS
 <-- A008 Expunge complete
 {Wait for command}
 A009 LOGOUT -->
 <-- A009 BYE IMAP3 server quitting
 <-- A009 OK Logout complete
 {Close Connection} --><-- {Close connection}
 {Go back to start}
 A more complex scenario produced by a pipelining multiprocess client.
 Client Server
 ------ ------
 {Wait for Connection}
 {Open session as above}
 <-- A004 19 EXISTS
 <-- A004 2 RECENT
 <-- A004 OK Select complete
 {Wait for command}
 A005 SEARCH RECENT -->
 <-- A005 SEARCH (18 19) (RECENT)
 <---A005 OK Search complete
 A006 FETCH 18:19 ALL RFC822.TEXT
 A007 STORE 18:19 +FLAGS (\SEEN)
 A008 FETCH 1:17 ALL -->
 <-- A006 18 Fetch (... RFC822.TEXT ...)
 A009 STORE 18 +FLAGS (\DELETED)
 <-- A006 19 Fetch (... RFC822.TEXT ...)
 <-- A006 OK Fetch complete
 <-- A007 18 STORE (Flags (\Seen))
 A010 STORE 19 +FLAGS (\DELETED)
 <-- A007 19 STORE (Flags (\Seen))
 <-- A007 OK Store complete
 <-- A008 1 Fetch (......)
 ...
 <-- A008 16 Fetch (......)
 <-- A008 17 Fetch (......)
 <-- A008 OK Fetch complete
 <-- A009 18 STORE (Flags (\Seen
 \Deleted))
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RFC 1203 IMAP3 February 1991
 <-- A009 OK Store complete
 <-- A010 19 STORE (Flags (\Seen
 \Deleted))
 <-- A010 OK Store complete
 {Wait for command}
 <-- * EXISTS 23
 <-- * RECENT 4
 <-- * SEARCH (20 21 22 23) (RECENT)
 A011 FETCH 20:23 ALL RFC822.TEXT
Conventions
 The following terms are used in a meta-sense in the syntax
 specification below:
 An ASCII-STRING is a sequence of arbitrary ASCII characters.
 An ATOM is a sequence of ASCII characters delimited by SP or CRLF.
 A CHARACTER is any ASCII character except """", "{", CR, LF, "%",
 or "\".
 A CRLF is an ASCII carriage-return character followed immediately
 by an ASCII linefeed character.
 A NUMBER is a sequence of the ASCII characters which represent
 decimal numerals ("0" through "9"), delimited by SP, CRLF, ",", or
 ":".
 A SP is the ASCII space character.
 A TEXT_LINE is a human-readable sequence of ASCII characters up to
 but not including a terminating CRLF.
 One of the most common fields in the IMAP3 protocol is a STRING,
 which may be an ATOM, QUOTED-STRING (a sequence of CHARACTERs inside
 double-quotes), or a LITERAL. A literal consists of an open brace
 ("{"), a number, a close brace ("}"), a CRLF, and then an ASCII-
 STRING of n characters, where n is the value of the number inside the
 brace. In general, a string should be represented as an ATOM or
 QUOTED-STRING if at all possible. The semantics for QUOTED-STRING or
 LITERAL are checked before those for ATOM; therefore an ATOM used in
 a STRING may only contain CHARACTERs. Literals are most often sent
 from the server to the client; in the rare case of a client to server
 literal there is a special consideration (see the "+ text" response
 below).
 Another important field is the SEQUENCE, which identifies a set of
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 messages by consecutive numbers from 1 to n where n is the number of
 messages in the mailbox. A sequence may consist of a single number,
 a pair of numbers delimited by colon indicating all numbers between
 those two numbers, or a list of single numbers and/or number pairs.
 For example, the sequence 2,4:7,9,12:15 is equivalent to
 2,4,5,6,7,9,12,13,14,15 and identifies all of those messages.
Definitions of Commands and Responses
 Summary of Commands and Responses
Commands:
 tag NOOP
 tag LOGIN user password
 tag LOGOUT
 tag SELECT mailbox
 tag CHECK
 tag EXPUNGE
 tag COPY sequence mailbox
 tag FETCH sequence data
 tag STORE sequence data value
 tag SEARCH criteria
 tag BBOARD bboard
 tag FIND (BBOARDS / MAILBOXES) pattern
 tag READONLY
 tag READWRITE
 tag SELECT.VERSION (major_version minor_version)
 tag SELECT.FEATURES features
 tag SUPPORTED.VERSIONS
 tag FLAGS
 tag SET.FLAGS
Responses (can be either solicited or unsolicited):
 */tag FLAGS flag_list
 */tag SEARCH (numbers) (criteria)
 */tag EXISTS
 */tag RECENT
 */tag EXPUNGE
 */tag STORE data
 */tag FETCH data
 */tag BBOARD bboard_name
 */tag MAILBOX non_inbox_mailbox_name
 */tag SUPPORTED.VERSIONS version_data
 */tag READONLY
 */tag READWRITE
 */tag OK text
 */tag NO text
 */tag BAD text
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 */tag BYE text
Responses (can only be solicited):
 tag COPY message_number
Responses (can only be unsolicited):
 + text
Commands
 tag NOOP
 The NOOP command returns an OK to the client. By itself, it does
 nothing, but certain things may happen as side effects. For
 example, server implementations which implicitly check the mailbox
 for new mail may do so as a result of this command. The primary
 use of this command is to for the client to see if the server is
 still alive (and notify the server that the client is still alive,
 for those servers which have inactivity autologout timers).
 tag LOGIN user password
 The LOGIN command identifies the user to the server and carries
 the password authenticating this user. This information is used
 by the server to control access to the mailboxes.
 EXAMPLE: A001 LOGIN SMITH SESAME logs in as user SMITH with
 password SESAME.
 tag LOGOUT
 The LOGOUT command indicates the client is done with the session.
 The server sends a solicited BYE response before the (tagged) OK
 response, and then closes the connection.
 tag SELECT mailbox
 The SELECT command selects a particular mailbox. The server must
 check that the user is permitted read access to this mailbox.
 Prior to returning an OK to the client, the server must send an
 solicited FLAGS and <n> EXISTS response to the client giving the
 flags list for this mailbox (simply the system flags if this
 mailbox doesn't have any special flags) and the number of messages
 in the mailbox. It is also recommended that the server send a <n>
 RECENT unsolicited response to the client for the benefit of
 clients which make use of the number of new messages in a mailbox.
 It is further recommended that servers should send an unsolicited
 READONLY message if the mailbox that has been selected is not
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 writable by the user.
 Multiple SELECT commands are permitted in a session, in which case
 the prior mailbox is deselected first.
 The default mailbox for the SELECT command is INBOX, which is a
 special name reserved to mean "the primary mailbox for this user
 on this server". The format of other mailbox names is operating
 system dependent (as of this writing, it reflects the path of the
 mailbox on the current servers), though it could reflect any
 server-specific naming convention for the namespace of mailboxes.
 Such a namespace need not and should not be viewed as being
 equivalent or linked to the server machine's file system.
 EXAMPLES: A002 SELECT INBOX ;; selects the default mailbox.
 A002 197 EXISTS ;; server says 197 messages in INBOX
 A002 5 RECENT ;; server says 5 are recent.
 A002 OK Select complete.
 or
 A003 SELECT /usr/fred/my-mail.txt
 ;; select a different user specified mailbox.
 ...
 tag CHECK
 The CHECK command forces a check for new messages and a rescan of
 the mailbox for internal change for those implementations which
 allow multiple simultaneous read/write access to the same mailbox
 (e.g., TOPS-20). It is recommend that periodic implicit checks
 for new mail be done by servers as well. The server must send a
 solicited <n> EXISTS response prior to returning an OK to the
 client.
 tag EXPUNGE
 The EXPUNGE command permanently removes all messages with the
 \DELETED flag set in its flags from the mailbox. Prior to
 returning an OK to the client, for each message which is removed,
 a solicited <n> EXPUNGE response is sent indicating which message
 was removed. The message number of each subsequent message in the
 mailbox is immediately decremented by 1; this means that if the
 last 5 messages in a 9-message mailbox are expunged you will
 receive 5 "5 EXPUNGE" responses for message 5. To ensure mailbox
 integrity and server/client synchronization, it is recommended
 that the server do an implicit check prior to commencing the
 expunge and again when the expunge is completed. Furthermore, if
 the server allows multiple simultaneous access to the same mailbox
 the server must guarantee both the integrity of the mailbox and
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 the views of it held by the clients.
 EXPUNGE is not allowed if the user does not have write access to
 this mailbox. If a user does not have write access to the mailbox
 then the server is required to signal this fact by replying with a
 NO response with a suitable text string that can be presented to
 the user explaining that the mailbox is read-only. It is further
 recommended that servers send an unsolicited READONLY message to
 clients that attempt an expunge operation on a read only mailbox.
 tag COPY sequence mailbox
 The COPY command copies the specified message(s) to the specified
 destination mailbox. If the destination mailbox does not exist,
 the server should create it. Prior to returning an OK to the
 client, the server must return a solicited <n> COPY response for
 each message copied.
 EXAMPLE: A003 COPY 2:4 MEETING copies messages 2, 3, and 4 to
 mailbox "MEETING".
 COPY is not allowed if the user does not have write access to the
 destination mailbox. If a user does not have write access to the
 destination mailbox then the server is required to signal this
 fact by replying with a NO response with a suitable text string
 that can be presented to the user explaining that the mailbox is
 read-only. It is further recommended that servers send an
 unsolicited READONLY message to clients that attempt to copy to a
 read only mailbox. IMAP3 does not specify "where" the message
 will be put in the mailbox to which it has been copied.
 tag FETCH sequence fetch_att
 The FETCH command retrieves data associated with a message in the
 mailbox. The data items to be fetched may be either a single atom
 or an S-expression list. The attributes that can be fetched are
 any of those mentioned specifically below along with any generic,
 canonical or concrete key. The set of predefined generic keys is:
 {BCC, BODY, CC, FROM, HEADER, SIZE, SUBJECT, TEXT, TO}. The set
 of predefined canonical keys is {$CC, $FROM, $SUBJECT, $TO}. The
 value returned by the server for a non-existent or non-meaningful
 key is defined to be the null value, NIL.
 ALL Equivalent to:
 (FLAGS INTERNALDATE RFC822.SIZE ENVELOPE)
 ENVELOPE The envelope of the message. The envelope is
 computed by the server by parsing the header,
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 i.e., the RFC 822 header for an RFC822 format
 message, into the component parts, defaulting
 various fields as necessary.
 FAST Macro equivalent to:
 (FLAGS INTERNALDATE RFC822.SIZE)
 FLAGS The flags which are set for this message.
 This may include the following system flags:
 \RECENT Message arrived since
 last read of this mailbox
 \SEEN Message has been read
 \ANSWERED Message has been answered
 \FLAGGED Message is "flagged" for
 urgent/special attention
 \DELETED Message is "deleted" for
 removal by later EXPUNGE
 INTERNALDATE The date and time the message was written to
 the mailbox.
 RFC822 The message in RFC 822 format.
 RFC822.HEADER The RFC 822 format header of the message.
 RFC822.SIZE The number of characters in the message as
 expressed in RFC 822 format.
 RFC822.TEXT The text body of the message, omitting the
 RFC 822 header.
 EXAMPLES:
 A003 FETCH 2:4 ALL
 fetches the flags, internal date, RFC 822 size, and envelope
 for messages 2, 3, and 4.
 A004 FETCH 3 RFC822
 fetches the RFC 822 representation for message 3.
 A005 FETCH 4 (FLAGS RFC822.HEADER)
 fetches the flags and RFC 822 format header for message 4.
 A006 FETCH 42 $SUBJECT
 A006 FETCH $SUBJECT "Some subject text..."
 A006 OK FETCH completed ok.
 fetches the canonical subject field.
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 A007 FETCH 42 APPARENTLY-TO
 A007 FETCH APPARENTLY-TO NIL
 A007 OK FETCH found no value.
 fetches the concrete apparently-to field.
 tag STORE sequence data value
 The STORE command alters the values associated with particular
 keys for a message in the mailbox. As is the case for the FETCH
 command, any generic, canonical or concrete key may be used to
 index the value provided. In addition to these, the following
 pre-defined keys are provided.
 FLAGS Replace the flags for the message with the
 argument (in flag list format).
 The server must respond with a solicited STORE FLAGS
 message, showing the new state of the flags after
 the store.
 +FLAGS Add the flags in the argument to the
 message's flag list.
 The server must respond with a solicited STORE FLAGS
 message, showing the new state of the flags after
 the store.
 -FLAGS Remove the flags in the argument from the
 message's flag list.
 The server must respond with a solicited STORE FLAGS
 message, showing the new state of the flags after
 the store.
 RFC822.HEADER Replace the header of the message(s) with that
 specified. This allows users to use their mailboxes
 as databases with header fields as keys.
 The server must respond with solicited
 STORE RFC822.HEADER, STORE RFC822.SIZE and
 STORE ENVELOPE messages, showing the new state
 of the reparsed header after the store.
 RFC822.TEXT Replace the body of the messages with that specified.
 The server must respond with solicited
 STORE RFC822.TEXT and STORE RFC822.SIZE messages,
 showing the new state of the message after the store.
 STORE is not allowed if the user does not have write access to
 this mailbox.
 The server is required to send a solicited STORE response for
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 each store operation that results in a format transformation by
 the server. For example, the server is required to send a
 STORE FLAGS response when the client performs a STORE +FLAGS or
 a STORE -FLAGS, since the client may not easily be able to know
 what the result of this command will be. Similarly, if the
 client emits a STORE FROM command then the server should
 respond with a suitable STORE FROM response because the client
 would be sending a string value to be stored and the server
 should transform this into a set of addresses. In general,
 however, although it is legal for the server to send a
 solicited STORE response for each STORE operation, this is
 discouraged, since it might result in the retransmission of
 very large and unnecessary amounts of data that have been
 stored.
 EXAMPLE: A003 STORE 2:4 +FLAGS (\DELETED) marks messages 2, 3,
 and 4 for deletion.
 tag SEARCH search_criteria
 The SEARCH command searches the mailbox for messages which match
 the given set of criteria. The server response SEARCH (criteria)
 (numbers) gives the set of messages which match the conjunction of
 the criteria specified. In addition to each of the search
 criteria there is its logical inverse. The logical inverse
 criterion is denoted by the ~ (tilda) sign.
 Thus, no message that matches the criterion:
 FROM crispin
 will match the criterion:
 ~FROM crispin
 The criteria for the search can be any generic, canonical or
 concrete key. In addition to these, the following pre-defined
 keys are also provided:
 ALL All messages in the mailbox; the default
 initial criterion for ANDing.
 ANSWERED Messages with the \ANSWERED flag set.
 BCC string Messages which contain the specified string
 in the envelope's BCC field.
 BEFORE date Messages whose internal date is earlier than
 the specified date.
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 BODY string Messages which contain the specified string
 in the body of the message.
 CC string Messages which contain the specified string
 in the envelope's CC field.
 DELETED Messages with the \DELETED flag set.
 FLAGGED Messages with the \FLAGGED flag set.
 FROM string Messages which contain the specified string
 in the envelope's FROM field.
 HEADER string Messages which contain the specified string
 in the message header.
 KEYWORD flag Messages with the specified flag set.
 NEW Messages which have the \RECENT flag set but
 not the \SEEN flag. This is functionally
 equivalent to "RECENT UNSEEN".
 OLD Messages which do not have the \RECENT flag
 set.
 ON date Messages whose internal date is the same as
 the specified date.
 RECENT Messages which have the \RECENT flag set.
 SEEN Messages which have the \SEEN flag set.
 SINCE date Messages whose internal date is later than
 the specified date.
 SUBJECT string Messages which contain the specified string
 in the envelope's SUBJECT field.
 TEXT string Messages which contain the specified string.
 TO string Messages which contain the specified string in
 the envelope's TO field.
 EXAMPLE: A003 SEARCH DELETED FROM "SMITH" SINCE 1-OCT-87
 returns the message numbers for all deleted messages from Smith
 that were placed in the mailbox since October 1, 1987.
 Implementation note: The UNANSWERED, UNDELETED, UNFLAGGED,
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 UNKEYWORD and UNSEEN criteria, described below, are preserved in
 IMAP3 for IMAP2 compatibility. They are, however, considered
 obsolete and new Client programs are encouraged to use the ~
 notation for the logical inverses of search criteria with a view
 to the dropping of this outmoded syntax in later versions.
 UNANSWERED Messages which do not have the \ANSWERED flag
 set.
 UNDELETED Messages which do not have the \DELETED flag
 set.
 UNFLAGGED Messages which do not have the \FLAGGED flag
 set.
 UNKEYWORD flag Messages which do not have the specified flag
 set.
 UNSEEN Messages which do not have the \SEEN flag set.
 tag READONLY
 The READONLY command indicates that the client wishes to make the
 mailbox read-only. The server is required to reply with a
 solicited READONLY or READWRITE response.
 tag READWRITE
 The READWRITE command indicates that the client wishes to make the
 mailbox read-write. The server is required to reply with a
 solicited READONLY or READWRITE response.
 tag SUPPORTED.VERSIONS
 The SUPPORTED.VERSIONS solicits from the server a
 SUPPORTED.VERSIONS message, which encapsulates information about
 which versions and features the server supports.
 tag SELECT.VERSION (major_version minor_version)
 The SELECT.VERSION command indicates that the client wishes to
 select certain behavior on the part of the server. The major and
 minor versions indicate the specific version of the protocol being
 selected.
 EXAMPLE: A002 SELECT.VERSION (3 0)
 A client may not request a server version that is not supported by
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 the server, i.e., which is specifically mentioned in the response
 to a SUPPORTED.VERSIONS command. An attempt to do so by a client
 will result in a NO response from the server. It is an error for
 the SELECT.VERSION command to be used after a mailbox has been
 selected. The rationale for this is that for some server
 implementations it might be necessary to spawn separate programs
 to implement widely divergent protocol versions. Thus, the client
 cannot be allowed to expect any server state to be preserved after
 the use of the SELECT.VERSION command. The default version of all
 servers is 2.0, i.e., IMAP2 as defined by RFC 1064.
 tag SELECT.FEATURES 1#features
 The SELECT.FEATURES command indicates that the client wishes to
 select certain specific features on the part of the server. A
 client may not request a feature that is not supported by the
 server, i.e., one that is explicitly mentioned in the set of
 features for the selected version returned by the
 SUPPORTED.VERSIONS command. An attempt to do so by a client will
 result in a NO response from the server.
 EXAMPLE: A002 SELECT.FEATURES AUTO.SET.SEEN ~TAGGED.SOLICITED
 EIGHT.BIT.TRANSPARENT
 i.e., select the set of features called AUTO.SET.SEEN and
 EIGHT.BIT.TRANSPARENT and deselect the feature called
 TAGGED.SOLICITED. The use of the SELECT.FEATURES command
 completely resets the set of selected features. Note: These are
 only example feature names and are not necessarily supported by
 any server. See the appendix on features for more information on
 features. Note: Some features, when present in the server, will
 cause the upwards compatible extension of the grammar, i.e., by
 adding extra commands. The server is at liberty not to remove
 these upwards compatible extensions to the command tables when a
 feature is disabled. Thus, it is an error for a client to rely on
 getting a NO or BAD response in any way, for instance to determine
 the selectedness or presence of a feature.
 tag BBOARD bboard
 The BBOARD command is equivalent to SELECT, except that its
 argument is a bulletin board (BBoard) name. The format of a
 BBoard name is implementation specific, although it is strongly
 encouraged to use something that resembles a name in a generic
 sense and not a file or mailbox name on the particular system.
 There is no requirement that a BBoard name be a mailbox name or a
 file name (in particular, Unix netnews has a completely different
 namespace from mailbox or file names).
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 The result from the BBOARD command is identical from that of the
 SELECT command. For example, in the TOPS-20 server
 implementation, the command
 A0002 BBOARD FOO
 is exactly equivalent to the command
 A0002 SELECT POBOX:<BBOARD>FOO.TXT
 Note: the equivalence in this example is *not* required by the
 protocol, and merely reflects the fuzzy distinction between
 mailboxes and BBoards on TOPS-20.
 tag FIND (BBOARDS / MAILBOXES) pattern
 The FIND command accepts as arguments the keywords BBOARDS or
 MAILBOXES and a pattern which specifies some set of BBoard/mailbox
 names which are usable by the BBOARD/SELECT command. Two wildcard
 characters are defined; "*" specifies that any number (including
 zero) characters may match at this position and "%" specifies that
 a single character may match at this position. For example,
 FOO*BAR will match FOOBAR, FOOD.ON.THE.BAR and FOO.BAR, whereas
 FOO%BAR will match only FOO.BAR; furthermore, "*" will match all
 BBoards/mailboxes. The following quoting convention applies to
 wildcards: "\*" is the literal "*" character, "\%" is the literal
 "%" character and "\\" is the literal "\" character. Notes: The
 format of mailboxes is server implementation dependent. The
 special mailbox name INBOX is not included in the output to the
 FIND MAILBOXES command.
 The FIND command solicits any number of BBOARD or MAILBOX
 responses from the server as appropriate.
 Examples:
 A0002 FIND BBOARDS *
 A0002 BBOARD FOOBAR
 A0002 BBOARD GENERAL
 A0002 OK FIND completed
 or
 A0002 FIND MAILBOXES FOO%BA*
 A0002 MAILBOX FOO.BAR
 A0002 MAILBOX FOO.BAZZAR
 A0002 OK FIND completed
 Note: Although the use of explicit file or path names for
 mailboxes is discouraged by this standard, it may be unavoidable.
 It is important that the value returned in the MAILBOX solicited
 reply be usable in the SELECT command without remembering any path
 specification which may have been used in the FIND MAILBOXES
 pattern.
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 tag FLAGS
 The FLAGS command solicits a FLAGS response from the server.
 tag SET.FLAGS flag_list
 The SET.FLAGS command defines the user specifiable flags for this
 mailbox, i.e., the keywords. If this set does not include flags
 formerly sent to the client by the server in a FLAGS message then
 this constitutes a request to delete the flag. Any new flags
 should be created. This command does not affect the system
 defined flags and any system flags that are included in the
 flag_list will be ignored. The server must respond to this
 command with a solicited FLAGS message. If the deletion of a flag
 results in the invalidation of the flag sets of any messages then
 the server is required to send solicited STORE FLAGS messages to
 the client for each modified message.
Responses:
 */tag OK text
 In its solicited form this response identifies successful
 completion of the command with the indicated tag. The text is a
 line of human-readable text which may be useful in a protocol
 telemetry log for debugging purposes.
 In its unsolicited form, this response indicates simply that the
 server is alive. No special action on the part of the client is
 called for. This is presently only used by servers at startup as
 a greeting message indicating that they are ready to accept the
 first command. This usage, although legal, is by no means
 required. The text is a line of human-readable text which may be
 logged in protocol telemetry.
 */tag NO text
 In its solicited form this response identifies unsuccessful
 completion of the command with the indicated tag. The text is a
 line of human-readable text which probably should be displayed to
 the user in an error report by the client.
 In its unsolicited form this response indicates some operational
 error at the server which cannot be traced to any protocol
 command. The text is a line of human-readable text which should
 be logged in protocol telemetry for the maintainer of the server
 and/or the client.
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 */tag BAD text
 In its solicited form response indicates faulty protocol received
 from the client and indicates a bug. The text is a line of
 human-readable text which should be recorded in any telemetry as
 part of a bug report to the maintainer of the client.
 In its unsolicited form response indicates some protocol error at
 the server which cannot be traced to any protocol command. The
 text is a line of human-readable text which should be logged in
 protocol telemetry for the maintainer of the server and/or the
 client. This generally indicates a protocol synchronization
 problem, and examination of the protocol telemetry is advised to
 determine the cause of the problem.
 */tag BYE text
 This indicates that the server is about to close the connection.
 The text is a line of human-readable text which should be
 displayed to the user in a status report by the client. IMAP2
 requires that the server emit a solicited BYE response as part of
 a normal logout sequence. This solicited form is not required
 under IMAP3, though is still legal for compatibility. In its
 unsolicited form the BYE response is used as a panic shutdown
 announcement by the server. It is required to be used by any
 server which performs autologouts due to inactivity.
 */tag number message_data
 The solicited (tag number message_data) response is generated as
 the result of a number of client requests. The server may also
 emit any the following at any time as unsolicited data (i.e., *
 number message_data). The message_data is one of the following:
 EXISTS The specified number of messages exists in the mailbox.
 RECENT The specified number of messages have arrived since the
 last time this mailbox was selected with the SELECT
 command or equivalent.
 EXPUNGE The specified message number has been permanently
 removed from the mailbox, and the next message in the
 mailbox (if any) becomes that message number.
 The server must send a solicited EXPUNGE response
 for each message that it expunges as the result
 of an EXPUNGE command. Note: future versions of the
 protocol may allow the use of a message sequence
 as a value returned by the EXPUNGE response to allow the
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 more efficient compaction of client representations of
 mailboxes.
 STORE data
 Functionally equivalent to FETCH, only it is sent by the
 server when the state of a mailbox changes. The server
 must send solicited STORE responses as the result of
 any change caused by a STORE command.
 FETCH data
 This is the principle means by which data about a
 message is sent to the client. The data is in a
 Lisp-like S-expression property list form. Just as the
 FETCH request from the client can fetch any generic,
 canonical or concrete key, so also the FETCH response
 can return values for any of these keys as well as for
 the pre-defined attributes mentioned below. Note that
 the server is permitted to send any unsolicited FETCH
 or STORE messages that it should choose, be they the
 values associated with generic, canonical or concrete
 keys. Clients are required to ignore any such
 FETCH responses that it cannot interpret. For example,
 clients are not required to be able to understand, i.e.,
 use fruitfully, the canonical $TO key, but they are
 required to be able to ignore an unsolicited $TO message
 correctly.
 ENVELOPE An S-expression format list which describes the
 envelope of a message. The envelope is computed
 by the server by parsing the RFC 822 header into
 the component parts, defaulting various fields
 as necessary.
 The fields of the envelope are in the following
 order: date, subject, from, sender, reply-to, to,
 cc, bcc, in-reply-to, and message-id. The date,
 subject, in-reply-to, and message-id fields are
 strings. The from, sender, reply-to, to, cc,
 and bcc fields are lists of addresses.
 An address is an S-expression format list which
 describes an electronic mail address. The fields
 of an address are in the following order:
 personal name, source-route (i.e., the
 at-domain-list in SMTP), mailbox name, host name
 and comments. Implementation note: The addition
 of the comment field is an incompatible extension
 from IMAP2. The server is required not to provide
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 this field when running in IMAP2 mode.
 Any field of an envelope or address which is
 not applicable is presented as the atom NIL.
 Note that the server must default the reply-to
 and sender fields from the from field; a client is
 not expected to know to do this.
 FLAGS An S-expression format list of flags which are set
 for this message. This may include the following
 system flags:
 \RECENT Message arrived since last
 read of this mailbox
 \SEEN Message has been read
 \ANSWERED Message has been answered
 \FLAGGED Message is "flagged" for
 urgent/special attention
 \DELETED Message is "deleted" for
 removal by later EXPUNGE
 INTERNALDATE A string containing the date and time the
 message was written to the mailbox.
 RFC822 A string expressing the message in RFC 822
 format.
 Note: Some implementations of IMAP2 servers
 had the (undocumented) behavior of setting
 the \SEEN flag as a side effect of fetching
 the body of a message. This resulted in
 erroneous behavior for clients that prefetch
 messages that the user might not get
 around to reading. Thus, this behavior is
 explicitly disallowed in IMAP3.
 Note: this is not a significant performance
 restriction because it is always possible for
 IMAP3 clients to use an interaction with the
 server of the following type:
 A001 FETCH 42 RFC822
 A002 STORE 42 +FLAGS (\SEEN)
 A001 42 FETCH RFC822 {637} ......
 A001 OK Fetch completed
 A002 42 STORE FLAGS (\SEEN \FLAGGED...)
 A002 OK Store Completed.
 RFC822.HEADER A string expressing the RFC 822 format
 header of the message
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 RFC822.SIZE A number indicating the number of
 characters in the message as expressed
 in RFC 822 format.
 RFC822.TEXT A string expressing the text body of the
 message, omitting the RFC 822 header.
 See also note for RFC822.
 */tag FLAGS flag_list
 A solicited FLAGS response must occur as a result of a SELECT
 command. The flag list is the list of flags (at a minimum, the
 IMAP defined flags) which are applicable for this mailbox. Flags
 other than the system flags are a function of the server
 implementation.
 */tag SEARCH (numbers) (search_criteria)
 This response occurs as a result of a SEARCH command. The
 number(s) refer to those messages which match the search criteria.
 In its solicited form this message allows clients to find
 interesting groups of messages, e.g., unseen messages from
 Crispin. In its unsolicited form it allows the server to inform
 the client of interesting patterns, e.g., when new mail arrives,
 recent and from Crispin. Compatibility note: The search_criteria
 are sent by the server along with the matching numbers so
 unsolicited SEARCH messages may be interpreted. This syntax is
 not upwards compatible with IMAP2 and so the new syntax is
 intended to make it simple for clients that are not able to take
 advantage of unsolicited SEARCH messages still to interpret
 solicited SEARCH messages simply by ignoring everything that
 follows the list of numbers with minimal parsing. Such clients
 may not, however, simply discard the rest of the line because
 there might be LITERALs in the search pattern.
 Examples:
 A00042 SEARCH (2 3 6) (FROM Crispin ~SEEN)
 and
 * SEARCH (42) (FROM Crispin RECENT)
 */tag READONLY
 This indicates that the mailbox is read-only. The server is
 required to respond to a READONLY or READWRITE command with either
 a solicited READONLY or a solicited READWRITE response. Note: If
 the client attempts a mutation operation, such as STORE, on a
 mailbox to which it does not have write access then the server is
 required to reply with a solicited READONLY response on the first
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RFC 1203 IMAP3 February 1991
 such attempted mutation. The server may also choose to send
 solicited READONLY responses for each subsequent attempted
 mutation.
 */tag READWRITE
 This indicates that the mailbox is read-write. The server is
 required to respond to a READONLY or READWRITE command with either
 a solicited READONLY or a solicited READWRITE response.
 */tag BBOARD bboard_name
 This message is produced in its solicited form as a response to a
 FIND BBOARDS command. In its unsolicited form it represents a
 notification by the server that a new BBoard has been added.
 Bboard_name must be a name that can be supplied to the BBOARD
 command so as to select the appropriate bboard.
 */tag MAILBOX non_inbox_mailbox_name
 This message is produced in its solicited form as a response to a
 FIND MAILBOXES command. In its unsolicited form it represents a
 notification by the server that a new mailbox has been added,
 perhaps as the result of a COPY command creating a new mailbox.
 Non_inbox_mailbox_name must be a name that can be supplied to the
 SELECT command so as to select the appropriate mailbox. Note:
 non_inbox_mailbox_name is never the string "INBOX".
 */tag SUPPORTED.VERSIONS (version_specs)
 This message is used either as a response to the
 SUPPORTED.VERSIONS or, in its unsolicited form, to indicate the
 dynamic addition or removal of support for features or protocol
 versions. Each version_spec is of the form (4 2
 EIGHT.BIT.TRANSPARENT AUTO.SET.SEEN ...), i.e., a major version
 number and a minor version number for the protocol and the set of
 features supported under the server's implementation of that
 protocol version. A server may not dynamically remove support for
 any version or feature that has been selected by any currently
 logged in client by the use of the VERSION command.
 Example:
 A00005 SUPPORTED.VERSIONS ((2 0 )
 (2 2 TAGGED.SOLICITED)
 (3 0 EIGHT.BIT.TRANSPARENT TAGGED.SOLICITED))
 Indicates that two major versions are supported and one minor
 version is supported and that tagged solicited messages are
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RFC 1203 IMAP3 February 1991
 supported in versions 2.2 and 3.0 with eight bit characters being
 supported under version 3. For each feature mentioned in the list
 of features there is also always the negation of that feature.
 For example, if the server supports the TAGGED.SOLICITED feature
 then it also supports the ~TAGGED.SOLICITED feature, which
 disables this feature. Note: These are only example feature
 names and are not necessarily supported by any server. See the
 appendix on features for more information on features.
 + text
 This response indicates that the server is ready to accept the
 text of a literal from the client. Normally, a command from the
 client is a single text line. If the server detects an error in
 the command, it can simply discard the remainder of the line. It
 cannot do this in the case of commands which contain literals,
 since a literal can be an arbitrarily long amount of text, and the
 server may not even be expecting a literal. This mechanism is
 provided so the client knows not to send a literal until the
 server definitely expects it, preserving client/server
 synchronization.
 In actual practice, this situation is rarely encountered. In the
 current protocol, the only client commands likely to contain
 literals are the LOGIN command and the STORE RFC822.HEADER or
 STORE RFC822.TEXT commands. Consider a situation in which a
 server validates the user before checking the password. If the
 password contains "funny" characters and hence is sent as a
 literal, then if the user is invalid an error would occur before
 the password is parsed.
 No such synchronization protection is provided for literals sent
 from the server to the client, for performance reasons. Any
 synchronization problems in this direction would be due to a bug
 in the client or server and not for some operational problem.
Sample IMAP3 session
 The following is a transcript of an actual IMAP3 session. Server
 output is identified by "S:" and client output by "U:". In cases
 where lines were too long to fit within the boundaries of this
 document, the line was continued on the next line preceded by a tab.
 S: * OK SUMEX-AIM.Stanford.EDU Interactive Mail Access Protocol
 III Service 6.1(349) at Mon, 14 May 90 14:58:30 PDT
 U: a001 SUPPORTED.VERSIONS
 S: * SUPPORTED.VERSIONS ((2 0 ) (3 0 EIGHT.BIT.TRANSPARENT
 AUTO.SET.SEEN TAGGED.SOLICITED))
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RFC 1203 IMAP3 February 1991
 S: A001 Supported Versions returned.
 U: a002 SELECT.VERSION (3 0)
 S: a002 OK Version 3.0 Selected.
 U: a003 SELECT.FEATURES TAGGED.SOLICITED
 S: a003 OK Features selected.
 U: a004 login crispin secret
 S: a004 OK User CRISPIN logged in at Thu, 9 Jun 90 14:58:42 PDT,
 job 76
 U: a005 select inbox
 S: a005 FLAGS (Bugs SF Party Skating Meeting Flames Request AI
 Question Note \XXXX \YYYY \Answered \Flagged \Deleted
 \Seen)
 S: a005 16 EXISTS
 S: a005 0 RECENT
 S: a006 OK Select complete
 U: a006 fetch 16 all
 S: a006 16 Fetch (Flags (\Seen) InternalDate " 9-Jun-88 12:55:
 RFC822.Size 637 Envelope ("Sat, 4 Jun 88 13:27:11 PDT"
 "INFO-MAC Mail Message" (("Larry Fagan" NIL "FAGAN"
 "SUMEX-AIM.Stanford.EDU" NIL)) (("Larry Fagan" NIL "FAGAN"
 "SUMEX-AIM.Stanford.EDU" NIL)) (("Larry Fagan" NIL "FAGAN"
 "SUMEX-AIM.Stanford.EDU" NIL)) ((NIL NIL "rindflEISCH"
 "SUMEX-AIM.Stanford.EDU" NIL)) NIL NIL NIL
 "<12403828905.13.FAGAN@SUMEX-AIM.Stanford.EDU>"))
 S: a006 OK Fetch completed
 U: a007 fetch 16 rfc822
 S: a007 16 Fetch (RFC822 {637}
 S: Mail-From: RINDFLEISCH created at 9-Jun-88 12:55:43
 S: Mail-From: FAGAN created at 4-Jun-88 13:27:12
 S: Date: Sat, 4 Jun 88 13:27:11 PDT
 S: From: Larry Fagan <FAGAN@SUMEX-AIM.Stanford.EDU>
 S: To: rindflEISCH@SUMEX-AIM.Stanford.EDU
 S: Subject: INFO-MAC Mail Message
 S: Message-ID: <12403828905.13.FAGAN@SUMEX-AIM.Stanford.EDU>
 S: ReSent-Date: Thu, 9 Jun 88 12:55:43 PDT
 S: ReSent-From: TC Rindfleisch <Rindfleisch@SUMEX-AIM.Stanford.EDU>
 S: ReSent-To: Yeager@SUMEX-AIM.Stanford.EDU,
 Crispin@SUMEX-AIM.Stanford.EDU
 S: ReSent-Message-ID:
 <12405133897.80.RINDFLEISCH@SUMEX-AIM.Stanford.EDU>
 S:
 S: The file is <info-mac>usenetv4-55.arc ...
 S: Larry
 S: -------
 S: )
 S: a007 OK Fetch completed
 U: a008 logout
 S: a008 BYE UNIX IMAP III server terminating connection
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RFC 1203 IMAP3 February 1991
 S: a008 OK SUMEX-AIM.Stanford.EDU Interim Mail Access Protocol
 Service logout
Implementation Discussion
 As of this writing, SUMEX has completed an IMAP2 client for Xerox
 Lisp machines written in hybrid Interlisp/CommonLisp and is beginning
 distribution of a client for TI Explorer Lisp machines. SUMEX has
 also completed a portable IMAP2 client protocol library module
 written in C. This library, with the addition of a small main
 program (primarily user interface) and a TCP/IP driver, became a
 rudimentary remote system mail-reading program under Unix. The first
 production use of this library is as a part of a MacII client which
 has now been under daily use (by real users) at Stanford for quite
 some time.
 As of this writing, SUMEX has completed IMAP2 servers for TOPS-20
 written in DEC-20 assembly language and 4.2/3 BSD Unix written in C.
 The TOPS-20 server is fully compatible with MM-20, the standard
 TOPS-20 mailsystem, and requires no special action or setup on the
 part of the user. The INBOX under TOPS-20 is the user's MAIL.TXT.
 The TOPS-20 server also supports multiple simultaneous access to the
 same mailbox, including simultaneous access between the IMAP3 server
 and MM-20. The 4.2/3 BSD Unix server requires that the user use
 either Unix Mail format or mail.txt format which is compatible with
 SRI MM-32 or Columbia MM-C. The 4.2/3 BSD Unix server allows
 simultaneous read access; write access must be exclusive. There is
 also an experimental IMAP3 server running on the TI Explorer class of
 machine, which uses MM mailbox format and which can communicate over
 both TCP and Chaos.
 The Xerox Lisp client and DEC-20 server have been in production use
 for over two years; the Unix server was been in production use for
 over a year. IMAP3 has been used to access mailboxes at remote sites
 from a local workstation via the Internet. For example, from the
 Stanford local network one of the authors has read his mailbox at a
 Milnet site.
 A number of IMAP clients have now been developed or are being
 developed. Amongst these are versions that run on the following
 machines:
 . Xerox Lisp machines
 . Apple Macintosh
 . NeXT
 . IBM PC
 . TI Explorer Lisp machines
 . "Glass teletype" version that runs under Unix
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RFC 1203 IMAP3 February 1991
 . GNU Emacs
 . X Windows
 . NTT ELIS
 Each of these client programs is carefully tuned to optimize the
 performance and user interface in a manner that is consistent with
 the the user interface model of the native machine. For example, the
 Macintosh client features a "messy-desk" interface that allows the
 cutting and pasting of text with the use of the clipboard with a menu
 driven interface with keyboard accelerators.
 This specification does not make any formal definition of size
 restrictions, but some of the existing servers have the following
 limitations:
 DEC-20
 . length of a mailbox: 7,077,888 characters
 . maximum number of messages: 18,432 messages
 . length of a command line: 10,000 characters
 . length of the local host name: 64 characters
 . length of a "short" argument: 39 characters
 . length of a "long" argument: 491,520 characters
 . maximum amount of data output in a single fetch:
 655,360 characters
 TI-Explorer
 . length of a mailbox: limited by the Minimum of the size of the
 virtual address space and the size of the file system
 . maximum number of messages: limited by the the size of the
 virtual address space
 . length of a command line: limited by the the size of the
 virtual address space
 . length of the local host name: limited by the the size of the
 virtual address space
 . length of a "short" argument: limited by the the size of the
 virtual address space
 . length of a "long" argument: limited by the the size of the
 virtual address space
 . maximum amount of data output in a single fetch: not limited
 Typical values for these limits are 30Mb for file systems and 128Mb
 for virtual address space.
 To date, nobody has run up against any of these limitations, many of
 which are substantially larger than most current user mail reading
 programs.
 There are several advantages to the scheme of tags and solicited
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RFC 1203 IMAP3 February 1991
 responses and unsolicited data. First, the infamous synchronization
 problems of SMTP and similar protocols do not happen with tagged
 commands; a command is not considered satisfied until a completion
 acknowledgement with the same tag is seen. Tagging allows an
 arbitrary amount of other responses ("solicited" data) to be sent by
 the server with no possibility of the client losing synchronization.
 Compare this with the problems that FTP or SMTP clients have with
 continuation, partial completion, and commentary reply codes.
 Another advantage is that a non-lockstep client implementation is
 possible. The client could send a command, and entrust the handling
 of the server responses to a different process which would signal the
 client when the tagged response comes in. Some clients might be
 implemented in a thoroughly asynchronous manner, having, perhaps,
 multiple outstanding commands at any given time. Note: this does
 not require that the server process these commands in anything other
 than a lock-step manner. It simply allows clients to take advantage
 of servers that are able to do such asynchronous operations.
 It was observed that synchronization problems can occur with literals
 if the literal is not recognized as such. Fortunately, the cases in
 which this can happen are relatively rare; a mechanism (the special
 "+" tag response) was introduced to handle those few cases which
 could happen. The proper way to address this problem in all cases is
 probably to move towards a record-oriented architecture instead of
 the text stream model provided by TCP.
 Unsolicited data needs some discussion. Unlike most protocols, in
 which the server merely does the client's bidding, an IMAP3 server
 has a semi-autonomous role. By means of sending "unsolicited data",
 the server is in effect sending a command to the client -- to update
 and/or extend its (incomplete) model of the mailbox with new
 information from the server. In this viewpoint, although a "fetch"
 command is a request for specific information from the client, the
 server is always at liberty to include more than the desired data as
 "unsolicited". A server acknowledgement to the "fetch" is a
 statement that at least all the requested data has been sent.
 In terms of implementation, a simple lock-step client may have a
 local cache of data from the mailbox. This cache is incomplete in
 general, and at select time is empty. A listener on the IMAP
 connection in the client processes all solicited and unsolicited data
 symmetrically, and updates the cache based on this data, i.e., the
 client faults on a cache miss and asks the server to fill that cache
 slot synchronously. If a tagged completion response arrives, the
 listener unblocks the process which sent the tagged request.
 Clearly, given this model it is not strictly necessary to distinguish
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RFC 1203 IMAP3 February 1991
 most solicited from unsolicited data. Doing so, however, apart from
 being clearer, also allows such simplistic, lock-step client
 implementations that extract the specific value of the response to
 command by trapping the tagged response. This allows the client not
 to have to block on some complex predicate that involves watching to
 see an update in a cache cell.
 For example, perhaps as a result of opening a mailbox, solicited data
 from the server arrives. The first piece of data is the number of
 messages. This is used to size the cache; note that, if new mail
 arrives, by sending a new "number of messages" unsolicited data
 message server will cause the cache to be re-sized. If the client
 attempts to access information from the cache, it will encounter
 empty spots which will trigger "fetch" requests. The request would
 be sent, some solicited data including the answer to the fetch will
 flow back, and then the "fetch" response will unblock the client.
 People familiar with demand-paged virtual memory design will
 recognize this model as being very similar to page-fault handling on
 a demand-paged system.
Formal Syntax
 The following syntax specification uses the augmented Backus-Naur
 Form (BNF) notation as specified in RFC 822 with one exception; the
 delimiter used with the "#" construct is a single space (SP) and not
 a comma.
address ::= "(" addr_name SP addr_adl SP addr_mailbox SP
 addr_host addr_comment ")"
addr_adl ::= nil / string
addr_comment ::= nil / string
addr_host ::= nil / string
addr_mailbox ::= nil / string
addr_name ::= nil / string
bboard ::= "BBOARD" SP bboard_name
bboard_name ::= string
bboard_notify ::= "BBOARD" sp bboard_name
canonical_key ::= "$CC" / "$FROM" / "$SUBJECT" / "$TO"
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RFC 1203 IMAP3 February 1991
check ::= "CHECK"
concrete_key ::= string
copy ::= "COPY" SP sequence SP mailbox
criterion ::= "ALL" / "ANSWERED" /
 "BCC" SP string / "BEFORE" SP string /
 "BODY" SP string / "CC" SP string / "DELETED" /
 "FLAGGED" / "KEYWORD" SP atom / "NEW" / "OLD" /
 "ON" SP string / "RECENT" / "SEEN" /
 "SINCE" SP string / "TEXT" SP string /
 "TO" SP string / "UNANSWERED" / "UNDELETED" /
 "UNFLAGGED" / "UNKEYWORD" / "UNSEEN" / key SP string
criteria ::= 1#criterion
data ::= ("FLAGS" SP flag_list /
 search_notify / bboard_notify / mailbox_notify /
 supported_versions_notify / "READONLY" / "READWRITE" /
 "BYE" SP text_line / "OK" SP text_line /
 "NO" SP text_line / "BAD" SP text_line)
date ::= string in form "dd-mmm-yy hh:mm:ss-zzz"
envelope ::= "(" env_date SP env_subject SP env_from SP
 env_sender SP env_reply-to SP env_to SP
 env_cc SP env_bcc SP env_in-reply-to SP
 env_message-id ")"
env_bcc ::= nil / "(" 1*address ")"
env_cc ::= nil / "(" 1*address ")"
env_date ::= string
env_from ::= nil / "(" 1*address ")"
env_in-reply-to ::= nil / string
env_length ::= NUMBER
env_message-id ::= nil / string
env_reply-to ::= nil / "(" 1*address ")"
env_sender ::= nil / "(" 1*address ")"
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RFC 1203 IMAP3 February 1991
env_subject ::= nil / string
env_to ::= nil / "(" 1*address ")"
expunge ::= "EXPUNGE"
feature ::= ATOM
fetch ::= "FETCH" SP sequence SP ("ALL" / "FAST" /
 fetch_att / "(" 1#fetch_att ")")
fetch_att ::= "ENVELOPE" / "FLAGS" / "INTERNALDATE" /
 "RFC822" / "RFC822.HEADER" / "RFC822.SIZE" /
 "RFC822.TEXT" / key
find ::= "FIND" ("BBOARDS" / "MAILBOXES") pattern
flag_list ::= ATOM / "(" 1#ATOM ")"
flags ::= "FLAGS"
generic_key ::= "BCC" / "BODY" / "CC" / "FROM" / "HEADER" / "SIZE" /
 "SUBJECT" / "TEXT" / "TO"
key ::= generic_key / canonical_key / concrete_key
literal ::= "{" NUMBER "}" CRLF ASCII-STRING
login ::= "LOGIN" SP userid SP password
logout ::= "LOGOUT"
mailbox ::= "INBOX" / string
mailbox_notify ::= MAILBOX non_inbox_mailbox_name
msg_copy ::= "COPY"
msg_data ::= (msg_exists / msg_recent / msg_expunge /
 msg_fetch / msg_copy)
msg_exists ::= "EXISTS"
msg_expunge ::= "EXPUNGE"
msg_fetch ::= ("FETCH" / "STORE") SP "(" 1#("ENVELOPE" SP
 env_length envelope / "FLAGS" SP "(" 1#(recent_flag
 flag_list) ")" / "INTERNALDATE" SP date /
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RFC 1203 IMAP3 February 1991
 "RFC822" SP string / "RFC822.HEADER" SP string /
 "RFC822.SIZE" SP NUMBER / "RFC822.TEXT" SP
 string / key SP string_list) ")"
msg_recent ::= "RECENT"
msg_num ::= NUMBER
nil ::= "NIL"
non_inbox_mailbox_name ::= string
noop ::= "NOOP"
numbers ::= 1#NUMBER
password ::= string
pattern ::= string
recent_flag ::= "\RECENT"
read_only ::= "READONLY"
read_write ::= "READWRITE"
ready ::= "+" SP text_line
request ::= tag SP (noop / login / logout / select / check /
 expunge / copy / fetch / store / search /
 select_version / select_features /
 supported_versions / bboard / find /
 read_only / read_write / flags / set_flags ) CRLF
response ::= tag SP ("OK" / "NO" / "BAD") SP text_line CRLF
search ::= "SEARCH" SP criteria
search_notify ::= "SEARCH" SP (numbers) SP (criteria)
select ::= "SELECT" SP mailbox
select_features ::= "SELECT.FEATURES" 1#feature
select_version ::= "SELECT.VERSION" SP "(" NUMBER SP NUMBER ")"
sequence ::= NUMBER / (NUMBER "," sequence) / (NUMBER ":"
 sequence)
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RFC 1203 IMAP3 February 1991
set_flags ::= "SET.FLAGS" SP flag_list
solicited ::= tag SP (msg_num SP msg_data / data /
 solicited_only) CRLF
solicited_only ::= {None currently defined}
store ::= "STORE" SP sequence SP store_att
store_att ::= ("+FLAGS" SP flag_list / "-FLAGS" SP flag_list /
 "FLAGS" SP flag_list / RFC822.TEXT SP string
 / RFC822.HEADER SP string / key SP string)
string ::= atom / """" 1*character """" / literal
string_list ::= string / ("(" 1#string ")")
supported_versions ::= "SUPPORTED.VERSIONS"
supported_versions_notify ::= "SUPPORTED.VERSIONS" "(" 1#version_spec
 ")"
system_flags ::= "\ANSWERED" SP "\FLAGGED" SP "\DELETED" SP
 "\SEEN"
tag ::= atom
unsolicited ::= "*" SP (msg_num SP msg_data / data) CRLF
userid ::= string
version_spec ::= "(" NUMBER SP NUMBER SP 1#feature ")"
Appendix: Features.
 In this section we outline the standard features that are supported
 by all IMAP3 servers and identify those features which are
 recommended or experimental. For each of these features the default
 setting is specified. This means that it is required of any server
 that supports a given feature to make the default enabledness of that
 feature as is specified below. It is required that for each feature
 supported by a server the inverse feature should also be supported.
 The inverse feature name shall always be defined as the feature name
 preceded by the "~" character. Thus, the AUTO.SET.SEEN feature is
 disabled by the ~AUTO.SET.SEEN feature.
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RFC 1203 IMAP3 February 1991
 Required Features:
 AUTO.SET.SEEN - When this features is enabled (default is disabled),
 the \\SEEN flag is set for all appropriate messages as a side
 effect of any of the following:
 FETCH of RFC822
 FETCH of RFC822.TEXT
 COPY
 Justification: This feature is provided for the use of clients
 that are unable to pipeline their commands effectively and
 communicate over high latency connections. When disabled,
 the server will not perform any such side effects. This feature
 is also provided so as to smooth the transition from IMAP2 to
 IMAP3.
 TAGGED.SOLICITED - When this feature is enabled (default is enabled
 for IMAP3, disabled for IMAP2 mode), solicited responses from
 the server will have the tag specified by the client.
 When this feature is disabled, solicited responses from the
 server will have the IMAP2 compatible tag "*", not the
 tag specified by the client.
 Justification: This feature is provided so as to smooth the
 transition from IMAP2 to IMAP3.
 Recommended Features.
 EIGHT.BIT.TRANSPARENT - When this feature is enabled
 (default is disabled), the server allows the transparent
 transmission of eight bit characters. When this feature is
 disabled, the value of any bit other than the least significant
 7 bits transmitted by the server is unspecified. If this
 feature is enabled, the characters that compose all command
 keywords specified in the IMAP3 grammar and all feature names
 use only their 7 least significant bits.
 Justification: This feature is provided for the purpose of
 supporting national character sets within messages, encoded
 languages such as Japanese Kanji characters and also of binary
 data, such as programs, graphics and sound.
 NEW.MAIL.NOTIFY - When this feature is enabled (default is
 disabled for compatibility with the majority of existing
 IMAP2 servers), the server will notify the client of the
 arrival of new mail in the currently selected mailbox
 using the appropriate RECENT and EXISTS unsolicited messages
 without the client needing to send periodic CHECK commands.
 Justification: This feature is provided to allow clients to
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RFC 1203 IMAP3 February 1991
 switch off any periodic polling strategy that they may use
 to look for new mail. Such polling unnecessarily uses bandwidth
 and can cause the interactive performance to degrade because
 the user can be kept waiting while some background process
 is doing a CHECK.
 SEND - When this feature is enabled (default is disabled) a new
 "SEND" command becomes available to the client. The SEND
 command instructs the server to send a message, rather
 than requiring the client to use its own, local message
 sending capability, for example. An example of of the
 send command might be as follows:
 tag42 SEND RFC822 {2083}
 From: James Rice <Rice@Sumex-Aim.Stanford.Edu>
 To:.....
 If the server is unable to parse the message being sent then
 it is required to issue a suitable NO notification to the client.
 If the message cannot be delivered for some reason then the
 server should send a suitable message to the FROM: address
 of the message detailing the delivery failure.
 When the SEND feature is enabled, the "send" production in
 the grammar is added and as defined below. The "send"
 request is added to the list of requests in the request
 production also as shown below:
 message_format ::= RFC822
 request ::= tag SP (noop / login / logout / select / check /
 expunge / copy / fetch / store / search /
 select_version / select_features /
 supported_versions / bboard / find /
 read_only / read_write / flags /
 set_flags / send) CRLF
 send ::= SEND SP message_format SP string
 Justification: This feature is provided so that mail can be
 sent by the same reliable server that is used for the storage
 of mail. This has, amongst others, the following benefits:
 - Single process clients need not be delayed by mail
 transmission.
 - Mail sent by the client will have the server named as the
 message's sender. This can be important because there are
 a lot of mailers that erroneously cause reply mail to be
 sent to the Sender, not the From or Reply-To address. Since
 the client in general is not listening for mail being sent
 to it directly this can cause mail to be lost.
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RFC 1203 IMAP3 February 1991
 - Clients can be written that do not have any native message
 sending capability.
 ADD.MESSAGE - When this feature is enabled (default is disabled)
 a new "ADD.MESSAGE" command becomes available to the client.
 The ADD.MESSAGE command instructs the server to add the
 specified message to the designated mailbox. This command
 can be thought of as being like a COPY command except in
 this case the message that is put in the designated mailbox
 is specified as a string, rather than as a message number to
 be copied from the currently selected mailbox. An example
 use of this command might be as follows:
 tag42 ADD.MESSAGE OUTGOING-MAIL RFC822 {2083}
 From: James Rice <Rice@Sumex-Aim.Stanford.Edu>
 To:.....
 This will have the effect of adding the message to the mailbox
 called OUTGOING-MAIL.
 If the server is unable to parse the message being added then
 it is required to issue a suitable NO notification to the client.
 When the ADD.MESSAGE feature is enabled, the "add_message"
 production in the grammar is added and as defined below.
 The "add_message" request is added to the list of requests
 in the request production also as shown below:
 add_message ::= ADD.MESSAGE SP mailbox SP format SP string
 message_format ::= RFC822
 request ::= tag SP (noop / login / logout / select / check /
 expunge / copy / fetch / store / search /
 select_version / select_features /
 supported_versions / bboard / find /
 read_only / read_write / flags / set_flags /
 add_message) CRLF
 Justification: This feature is provided so that clients can
 easily add mail to specific mailboxes. This allows clients
 to implement such behavior as outgoing mail storage (BCC)
 without the need to resort to mailing to special BCC mailboxes.
 RENUMBER - When this feature is enabled (default is disabled)
 the RENUMBER command becomes available to the client.
 The RENUMBER command will reorder the assignment of message
 numbers to the messages in the mailbox. If this results in a
 change to the association of any message number with any
 message then the server is required to send solicited RESET
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RFC 1203 IMAP3 February 1991
 responses to the client. The intent of this command is
 to allow users to view mailboxes in user-meaningful order
 efficiently. While the client could do the ordering,
 it would be less efficient in general. Note that the
 server may or may not change the actual storage of the
 messages and the ordering may or may not remain in effect
 after another mailbox is selected or the IMAP session is
 terminated. Informally, the syntax for the RENUMBER
 command is:
 tag RENUMBER field_name ordering_type
 this has the effect of changing the IMAP grammar to be
 as follows:
 ordering_type ::= DATE / NUMERIC / ALPHA
 renumber ::= RENUMBER SP field_name SP ordering_type
 request ::= tag SP (noop / login / logout / select / check /
 expunge / copy / fetch / store / search /
 select_version / select_features /
 supported_versions / bboard / find /
 read_only / read_write / flags / set_flags /
 renumber) CRLF
 For example:
 tag42 RENUMBER FROM ALPHA
 ;;;RENUMBER alphabetically by the from field
 tag42 RESET 10:20,49
 ;;;Messages 10 to 20 and 49 have changed
 tag42 OK RENUMBER finished. Sequence has changed
 tag43 FETCH ALL 10:20,49
 ;;;Client chooses to fetch the changed msgs.
 To support this the RESET message is defined as follows:
 */tag RESET message_sequence
 This solicited of unsolicited message from the server informs the
 client that it should flush any information that it has
 retained for the specified messages.
 Justification: This feature is provided so that clients can
 view mailboxes in an order that is convenient to the user.
 This is particularly important in the context of mailboxes
 that the user copies messages to from other mailboxes. This
 user-controlled filing process often does not happen in any
 well-defined order. Because messages in a mailbox are
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 implicitly ordered (usually by arrival date, though this is
 not a required ordering predicate), the user can be confused
 by the apparent order of messages in the mailbox. The
 addition of the RENUMBER command makes it unnecessary
 for the user to leave IMAP and use some other mail system to
 sort mailboxes.
 ENCODING - When this feature is enabled (default is disabled) a new
 generic key named ENCODING is defined. The value associated
 with the generic ENCODING key is a list of (tag encoding-type
 options...) lists that represent the ordered, possibly encoded
 body of the message. Each such list represents a segment of
 the body of the message and the way in which it is encoded.
 Any options that follow the encoding_type are further
 qualifiers that describe the format of the segment. Each tag
 is created by the server and is unique with respect to the
 other tags allocated for the other elements in the ENCODING
 list. The client may use the tags returned by the server as
 concrete keys to access a field which is encoded using the
 encoding type and options mentioned in the appropriate list.
 Thus:
 tag41 FETCH 196 ENCODING ; Client asks for encoding field of msg 196.
 tag41 FETCH ENCODING NIL ; Server replies. This message is not encoded.
 tag41 OK Fetch completed.
 tag42 FETCH 197 ENCODING ; Client asks for encoding field of msg 197.
 tag42 FETCH ENCODING ((G001 UUENCODE) (G002 HEX)) ; Server replies.
 tag42 OK Fetch completed.
 tag43 FETCH 197 G002 ; Client asks for field named G002
 tag43 FETCH G002 "A0 00 FF 13 42......." ; Server sends value of field.
 tag43 OK Fetch completed.
 or
 tag44 STORE 197 G002 "0A 00 FF 31 24......."
 ; Store back the segment with nibbles swapped
 Note: As a side-effect of enabling this feature, the generic key
 TEXT will be redefined so as to return only those body parts of a
 message that are of type TEXT. The concrete key RFC822.TEXT, on
 the other hand, would still return everything in the body of the
 message, even if it was full of strange, binary character
 sequences.
 When the client STOREs to a field denoted by one of the above tags
 the server will interpret the value being passed as being in the
 same format as is currently specified in the ENCODING field. The
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RFC 1203 IMAP3 February 1991
 server is not required to be able to reformat the data associated
 with the ENCODING tags if the client STOREs a new value for the
 ENCODING field. The interpretability of a message in the context
 of its ENCODING field is undefined if the client side-effects that
 ENCODING field, unless the client also STOREs new, reformatted
 values for the fields that have had their encoding changed.
 If the client stores a new value for the ENCODING field then the
 tags in the new value will be used to index the parts of the body.
 All tags in a client-STOREd ENCODING that are the same as those
 originally generated by the server in response to a FETCH ENCODING
 command are said still to denote the fields that they originally
 denoted, though possibly reordered. Any tags not originally
 defined by the server will denote new message parts, in the
 appropriate format, in the relative position specified. The
 exclusion of any tags that the server originally defined in a
 FETCH of the ENCODING field will indicate the deletion of that
 part of the message. Newly created message parts are undefined by
 default, so if the client fails to follow the STOREing of the
 ENCODING field with suitable STORE commands for the values
 associated with any newly created tags, these fields will contain
 the null value NIL.
 Justification: This feature is supplied so as to allow support
 for emergent multi-part and multi-media mail standards.
 INDEXABLE.FIELDS - When this feature is enabled (default is
 disabled) the grammar of fetch commands is changed to allow the
 client to select a specific subsequence from the field in
 question. For example:
 tag42 FETCH 197 BODY 2000:3999
 would fetch the second two thousand bytes of the body of message
 197. This feature allows resource limited clients to access
 small parts of large messages. The formal syntax for this is:
 fetch_att ::= "ENVELOPE" / "FLAGS" / "INTERNALDATE" /
 fetch_key / (fetch_key SP NUMBER ":" NUMBER)
 fetch_key ::= "RFC822" / "RFC822.HEADER" / "RFC822.SIZE" /
 "RFC822.TEXT" / key
 If the lower bound number (the number to the left of the colon)
 exceeds the maximum size of the field then the empty string is
 returned. If the upper bound exceeds the maximum size of the
 field but the lower bound does not then the server will return the
 remaining substring of the field after the lower bound. The
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RFC 1203 IMAP3 February 1991
 bounds specified are zero indexed into the fields and the bounds
 index fields by 8-bit bytes.
 Justification: This feature is provided so as to allow resource-
 limited clients to read very large messages and also to allow
 clients to improve interactive response for the reading of large
 messages by fetching the first "screen full" of data to display
 immediately and fetching the rest of the message in the
 background.
 SET.EOL - When enabled (default is disabled), this feature
 allows the new command SET.EOL to be available, changing the
 grammar as follows:
 character ::= "CR" / "LF" / number
 request ::= tag SP (noop / login / logout / select / check /
 expunge / copy / fetch / store / search /
 select_version / select_features /
 supported_versions / bboard / find /
 read_only / read_write / flags / set_flags /
 set_eol) CRLF
 set_eol ::= "SET.EOL" 1#character
 This has the effect of changing the end of line character sequence
 generated by the server for newlines within strings to the
 sequence of characters specified. The characters in the sequence
 can be either the specified symbolically named characters or a
 numerical value, specifying the decimal value of the character to
 use. Thus, if the client would like newlines in strings to be
 indicated by a carriage return followed by a control-d, the client
 would issue the following command:
 tag42 SET.EOL CR 4
 If the server is unable to support the combination of characters
 requested by the client as its end-of-line pattern it will reply
 with a NO response. This might be the case, for example, if a
 server is only able to generate its own native line feed pattern
 and the CRLF required by IMAP by default.
 The server is required to change any length denoting values, such
 as envelope byte counts for all future transactions to reflect the
 new eol setting. This change in reported sizes should apply to
 all generic size fetching keys, but not to concrete ones such as
 RFC822.SIZE, which by their very nature require a size measurement
 in RFC822 format, i.e., with CRLF as the end-of-line convention.
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RFC 1203 IMAP3 February 1991
 Justification: This feature is provided because frequently clients
 and servers might have end-of-line conventions other than the CRLF
 specified by RFC822. It is undesirable that the IMAP be linked
 too closely to RFC822 and selecting a different convention might
 allow substantial performance improvements in both clients and
 servers by saving either client, server or both from having to
 shuffle text around so as to add or remove non-local end-of-line
 sequences.
Acknowledgements:
 This text is based on RFC 1064 by Mark Crispin.
 The following have made major contributions to this proposed update
 to the IMAP2 protocol:
 James Rice <Rice@sumex-aim.stanford.edu>
 Richard Acuff <acuff@sumex-aim.stanford.edu>
 Bill Yeager <yeager@sumex-aim.stanford.edu>
 Christopher Lane <lane@sumex-aim.stanford.edu>
 Bjorn Victor <Bjorn.Victor@docs.uu.se>
 Additional input was also received from:
 Andrew Sweer <sweer@sumex-aim.stanford.edu>
 Tom Gruber <Gruber@sumex-aim.stanford.edu>
 Kevin Brock <Brock@Sumex-Aim.Stanford.Edu>
 Mark Crispin <MRC@cac.washington.edu>
Security Considerations
 Security issues are not discussed in this memo.
Author's Address
 James Rice
 Stanford University
 Knowledge Systems Laboratory
 701 Welch Road
 Building C
 Palo Alto, CA 94304
 Phone: (415) 723-8405
 EMail: RICE@SUMEX-AIM.STANFORD.EDU
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