Post Office Protocol: Version 3
RFC 1225

Network Working Group M. Rose
Request for Comments: 1225 Performance Systems International
Obsoletes: RFC 1081 May 1991
 Post Office Protocol - Version 3
Status of this Memo
 This memo suggests a simple method for workstations to dynamically
 access mail from a mailbox server. This RFC specifies an IAB
 standards track protocol for the Internet community, and requests
 discussion and suggestions for improvements. 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.
Overview
 This memo is a republication of RFC 1081 which was based on RFC 918
 (since revised as RFC 937). Although similar in form to the original
 Post Office Protocol (POP) proposed for the Internet community, the
 protocol discussed in this memo is similar in spirit to the ideas
 investigated by the MZnet project at the University of California,
 Irvine.
 Further, substantial work was done on examining POP in a PC-based
 environment. This work, which resulted in additional functionality
 in this protocol, was performed by the ACIS Networking Systems Group
 at Stanford University. The author gratefully acknowledges their
 interest.
Introduction
 On certain types of smaller nodes in the Internet it is often
 impractical to maintain a message transport system (MTS). For
 example, a workstation may not have sufficient resources (cycles,
 disk space) in order to permit a SMTP server and associated local
 mail delivery system to be kept resident and continuously running.
 Similarly, it may be expensive (or impossible) to keep a personal
 computer interconnected to an IP-style network for long amounts of
 time (the node is lacking the resource known as "connectivity").
 Despite this, it is often very useful to be able to manage mail on
 these smaller nodes, and they often support a user agent (UA) to aid
 the tasks of mail handling. To solve this problem, a node which can
 support an MTS entity offers a maildrop service to these less endowed
 nodes. The Post Office Protocol - Version 3 (POP3) is intended to
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RFC 1225 POP3 May 1991
 permit a workstation to dynamically access a maildrop on a server
 host in a useful fashion. Usually, this means that the POP3 is used
 to allow a workstation to retrieve mail that the server is holding
 for it.
 For the remainder of this memo, the term "client host" refers to a
 host making use of the POP3 service, while the term "server host"
 refers to a host which offers the POP3 service.
A Short Digression
 This memo does not specify how a client host enters mail into the
 transport system, although a method consistent with the philosophy of
 this memo is presented here:
 When the user agent on a client host wishes to enter a message
 into the transport system, it establishes an SMTP connection to
 its relay host (this relay host could be, but need not be, the
 POP3 server host for the client host).
 If this method is followed, then the client host appears to the MTS
 as a user agent, and should NOT be regarded as a "trusted" MTS entity
 in any sense whatsoever. This concept, along with the role of the
 POP3 as a part of a split-UA model is discussed later in this memo.
 Initially, the server host starts the POP3 service by listening on
 TCP port 110. When a client host wishes to make use of the service,
 it establishes a TCP connection with the server host. When the
 connection is established, the POP3 server sends a greeting. The
 client and POP3 server then exchange commands and responses
 (respectively) until the connection is closed or aborted.
 Commands in the POP3 consist of a keyword possibly followed by an
 argument. All commands are terminated by a CRLF pair.
 Responses in the POP3 consist of a success indicator and a keyword
 possibly followed by additional information. All responses are
 terminated by a CRLF pair. There are currently two success
 indicators: positive ("+OK") and negative ("-ERR").
 Responses to certain commands are multi-line. In these cases, which
 are clearly indicated below, after sending the first line of the
 response and a CRLF, any additional lines are sent, each terminated
 by a CRLF pair. When all lines of the response have been sent, a
 final line is sent, consisting of a termination octet (decimal code
 046, ".") and a CRLF pair. If any line of the multi-line response
 begins with the termination octet, the line is "byte-stuffed" by
 pre-pending the termination octet to that line of the response.
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 Hence a multi-line response is terminated with the five octets
 "CRLF.CRLF". When examining a multi-line response, the client checks
 to see if the line begins with the termination octet. If so and if
 octets other than CRLF follow, the the first octet of the line (the
 termination octet) is stripped away. If so and if CRLF immediately
 follows the termination character, then the response from the POP
 server is ended and the line containing ".CRLF" is not considered
 part of the multi-line response.
 A POP3 session progresses through a number of states during its
 lifetime. Once the TCP connection has been opened and the POP3
 server has sent the greeting, the session enters the AUTHORIZATION
 state. In this state, the client must identify itself to the POP3
 server. Once the client has successfully done this, the server
 acquires resources associated with the client's maildrop, and the
 session enters the TRANSACTION state. In this state, the client
 requests actions on the part of the POP3 server. When the client has
 finished its transactions, the session enters the UPDATE state. In
 this state, the POP3 server releases any resources acquired during
 the TRANSACTION state and says goodbye. The TCP connection is then
 closed.
The AUTHORIZATION State
 Once the TCP connection has been opened by a POP3 client, the POP3
 server issues a one line greeting. This can be any string terminated
 by CRLF. An example might be:
 S. +OK dewey POP3 server ready (Comments to: PostMaster@UDEL.EDU)
 Note that this greeting is a POP3 reply. The POP3 server should
 always give a positive response as the greeting.
 The POP3 session is now in the AUTHORIZATION state. The client must
 now issue the USER command. If the POP3 server responds with a
 positive success indicator ("+OK"), then the client may issue either
 the PASS command to complete the authorization, or the QUIT command
 to terminate the POP3 session. If the POP3 server responds with a
 negative success indicator ("-ERR") to the USER command, then the
 client may either issue a new USER command or may issue the QUIT
 command.
 When the client issues the PASS command, the POP3 server uses the
 argument pair from the USER and PASS commands to determine if the
 client should be given access to the appropriate maildrop. If so,
 the POP3 server then acquires an exclusive-access lock on the
 maildrop. If the lock is successfully acquired, the POP3 server
 parses the maildrop into individual messages (read note below),
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 determines the last message (if any) present in the maildrop that was
 referenced by the RETR command, and responds with a positive success
 indicator. The POP3 session now enters the TRANSACTION state. If
 the lock can not be acquired or the client should is denied access to
 the appropriate maildrop or the maildrop can't be parsed for some
 reason, the POP3 server responds with a negative success indicator.
 (If a lock was acquired but the POP3 server intends to respond with a
 negative success indicator, the POP3 server must release the lock
 prior to rejecting the command.) At this point, the client may
 either issue a new USER command and start again, or the client may
 issue the QUIT command.
 NOTE: Minimal implementations of the POP3 need only be
 able to break a maildrop into its component messages;
 they need NOT be able to parse individual messages.
 More advanced implementations may wish to have this
 capability, for reasons discussed later.
 After the POP3 server has parsed the maildrop into individual
 messages, it assigns a message-id to each message, and notes the size
 of the message in octets. The first message in the maildrop is
 assigned a message-id of "1", the second is assigned "2", and so on,
 so that the n'th message in a maildrop is assigned a message-id of
 "n". In POP3 commands and responses, all message-id's and message
 sizes are expressed in base-10 (i.e., decimal).
 It sets the "highest number accessed" to be that of the last message
 referenced by the RETR command.
 Here are summaries for the three POP3 commands discussed thus far:
 USER name
 Arguments: a server specific user-id (required)
 Restrictions: may only be given in the AUTHORIZATION
 state after the POP3 greeting or after an
 unsuccessful USER or PASS command
 Possible Responses:
 +OK name is welcome here
 -ERR never heard of name
 Examples:
 C: USER mrose
 S: +OK mrose is a real hoopy frood
 ...
 C: USER frated
 S: -ERR sorry, frated doesn't get his mail here
 PASS string
 Arguments: a server/user-id specific password (required)
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 Restrictions: may only be given in the AUTHORIZATION
 state after a successful USER command
 Possible Responses:
 +OK maildrop locked and ready
 -ERR invalid password
 -ERR unable to lock maildrop
 Examples:
 C: USER mrose
 S: +OK mrose is a real hoopy frood
 C: PASS secret
 S: +OK mrose's maildrop has 2 messages
 (320 octets)
 ...
 C: USER mrose
 S: +OK mrose is a real hoopy frood
 C: PASS secret
 S: -ERR unable to lock mrose's maildrop, file
 already locked
 QUIT
 Arguments: none
 Restrictions: none
 Possible Responses:
 +OK
 Examples:
 C: QUIT
 S: +OK dewey POP3 server signing off
The TRANSACTION State
 Once the client has successfully identified itself to the POP3 server
 and the POP3 server has locked and burst the appropriate maildrop,
 the POP3 session is now in the TRANSACTION state. The client may now
 issue any of the following POP3 commands repeatedly. After each
 command, the POP3 server issues a response. Eventually, the client
 issues the QUIT command and the POP3 session enters the UPDATE state.
 Here are the POP3 commands valid in the TRANSACTION state:
 STAT
 Arguments: none
 Restrictions: may only be given in the TRANSACTION state.
 Discussion:
 The POP3 server issues a positive response with a line
 containing information for the maildrop. This line is
 called a "drop listing" for that maildrop.
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 In order to simplify parsing, all POP3 servers are
 required to use a certain format for drop listings.
 The first octets present must indicate the number of
 messages in the maildrop. Following this is the size
 of the maildrop in octets. This memo makes no
 requirement on what follows the maildrop size.
 Minimal implementations should just end that line of
 the response with a CRLF pair. More advanced
 implementations may include other information.
 NOTE: This memo STRONGLY discourages
 implementations from supplying additional
 information in the drop listing. Other,
 optional, facilities are discussed later on
 which permit the client to parse the messages
 in the maildrop.
 Note that messages marked as deleted are not counted in
 either total.
 Possible Responses:
 +OK nn mm
 Examples:
 C: STAT
 S: +OK 2 320
 LIST [msg]
 Arguments: a message-id (optionally) If a message-id is
 given, it may NOT refer to a message marked as
 deleted.
 Restrictions: may only be given in the TRANSACTION state.
 Discussion:
 If an argument was given and the POP3 server issues a
 positive response with a line containing information
 for that message. This line is called a "scan listing"
 for that message.
 If no argument was given and the POP3 server issues a
 positive response, then the response given is
 multi-line. After the initial +OK, for each message
 in the maildrop, the POP3 server responds with a line
 containing information for that message. This line
 is called a "scan listing" for that message.
 In order to simplify parsing, all POP3 servers are
 required to use a certain format for scan listings.
 The first octets present must be the message-id of
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 the message. Following the message-id is the size of
 the message in octets. This memo makes no requirement
 on what follows the message size in the scan listing.
 Minimal implementations should just end that line of
 the response with a CRLF pair. More advanced
 implementations may include other information, as
 parsed from the message.
 NOTE: This memo STRONGLY discourages
 implementations from supplying additional
 information in the scan listing. Other, optional,
 facilities are discussed later on which permit
 the client to parse the messages in the maildrop.
 Note that messages marked as deleted are not listed.
 Possible Responses:
 +OK scan listing follows
 -ERR no such message
 Examples:
 C: LIST
 S: +OK 2 messages (320 octets)
 S: 1 120
 S: 2 200
 S: .
 ...
 C: LIST 2
 S: +OK 2 200
 ...
 C: LIST 3
 S: -ERR no such message, only 2 messages in
 maildrop
 RETR msg
 Arguments: a message-id (required) This message-id may
 NOT refer to a message marked as deleted.
 Restrictions: may only be given in the TRANSACTION state.
 Discussion:
 If the POP3 server issues a positive response, then the
 response given is multi-line. After the initial +OK,
 the POP3 server sends the message corresponding to the
 given message-id, being careful to byte-stuff the
 termination character (as with all multi-line
 responses).
 If the number associated with this message is higher
 than the "highest number accessed" in the maildrop, the
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 POP3 server updates the "highest number accessed" to
 the number associated with this message.
 Possible Responses:
 +OK message follows
 -ERR no such message
 Examples:
 C: RETR 1
 S: +OK 120 octets
 S: <the POP3 server sends the entire message here>
 S: .
 DELE msg
 Arguments: a message-id (required) This message-id
 may NOT refer to a message marked as deleted.
 Restrictions: may only be given in the TRANSACTION state.
 Discussion:
 The POP3 server marks the message as deleted. Any
 future reference to the message-id associated with the
 message in a POP3 command generates an error. The POP3
 server does not actually delete the message until the
 POP3 session enters the UPDATE state.
 If the number associated with this message is higher
 than the "highest number accessed" in the maildrop,
 the POP3 server updates the "highest number accessed"
 to the number associated with this message.
 Possible Responses:
 +OK message deleted
 -ERR no such message
 Examples:
 C: DELE 1
 S: +OK message 1 deleted
 ...
 C: DELE 2
 S: -ERR message 2 already deleted
 NOOP
 Arguments: none
 Restrictions: may only be given in the TRANSACTION state.
 Discussion:
 The POP3 server does nothing, it merely replies with a
 positive response.
 Possible Responses:
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 +OK
 Examples:
 C: NOOP
 S: +OK
 LAST
 Arguments: none
 Restrictions: may only be issued in the TRANSACTION state.
 Discussion:
 The POP3 server issues a positive response with a line
 containing the highest message number which accessed.
 Zero is returned in case no message in the maildrop has
 been accessed during previous transactions. A client
 may thereafter infer that messages, if any, numbered
 greater than the response to the LAST command are
 messages not yet accessed by the client.
 Possible Response:
 +OK nn
 Examples:
 C: STAT
 S: +OK 4 320
 C: LAST
 S: +OK 1
 C: RETR 3
 S: +OK 120 octets
 S: <the POP3 server sends the entire message
 here>
 S: .
 C: LAST
 S: +OK 3
 C: DELE 2
 S: +OK message 2 deleted
 C: LAST
 S: +OK 3
 C: RSET
 S: +OK
 C: LAST
 S: +OK 1
 RSET
 Arguments: none
 Restrictions: may only be given in the TRANSACTION
 state.
 Discussion:
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 If any messages have been marked as deleted by the POP3
 server, they are unmarked. The POP3 server then
 replies with a positive response. In addition, the
 "highest number accessed" is also reset to the value
 determined at the beginning of the POP3 session.
 Possible Responses:
 +OK
 Examples:
 C: RSET
 S: +OK maildrop has 2 messages (320 octets)
The UPDATE State
 When the client issues the QUIT command from the TRANSACTION state,
 the POP3 session enters the UPDATE state. (Note that if the client
 issues the QUIT command from the AUTHORIZATION state, the POP3
 session terminates but does NOT enter the UPDATE state.)
 QUIT
 Arguments: none
 Restrictions: none
 Discussion:
 The POP3 server removes all messages marked as deleted
 from the maildrop. It then releases the
 exclusive-access lock on the maildrop and replies as
 to the success of
 these operations. The TCP connection is then closed.
 Possible Responses:
 +OK
 Examples:
 C: QUIT
 S: +OK dewey POP3 server signing off (maildrop
 empty)
 ...
 C: QUIT
 S: +OK dewey POP3 server signing off (2 messages
 left)
 ...
Optional POP3 Commands
 The POP3 commands discussed above must be supported by all minimal
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 implementations of POP3 servers.
 The optional POP3 commands described below permit a POP3 client
 greater freedom in message handling, while preserving a simple POP3
 server implementation.
 NOTE: This memo STRONGLY encourages implementations to
 support these commands in lieu of developing augmented
 drop and scan listings. In short, the philosophy of
 this memo is to put intelligence in the part of the
 POP3 client and not the POP3 server.
 TOP msg n
 Arguments: a message-id (required) and a number. This
 message-id may NOT refer to a message marked as
 deleted.
 Restrictions: may only be given in the TRANSACTION state.
 Discussion:
 If the POP3 server issues a positive response, then
 the response given is multi-line. After the initial
 +OK, the POP3 server sends the headers of the message,
 the blank line separating the headers from the body,
 and then the number of lines indicated message's body,
 being careful to byte-stuff the termination character
 (as with all multi-line responses).
 Note that if the number of lines requested by the POP3
 client is greater than than the number of lines in the
 body, then the POP3 server sends the entire message.
 Possible Responses:
 +OK top of message follows
 -ERR no such message
 Examples:
 C: TOP 10
 S: +OK
 S: <the POP3 server sends the headers of the
 message, a blank line, and the first 10 lines
 of the body of the message>
 S: .
 ...
 C: TOP 100
 S: -ERR no such message
 RPOP user
 Arguments: a client specific user-id (required)
 Restrictions: may only be given in the AUTHORIZATION
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RFC 1225 POP3 May 1991
 state after a successful USER command; in addition,
 may only be given if the client used a reserved
 (privileged) TCP port to connect to the server.
 Discussion:
 The RPOP command may be used instead of the PASS
 command to authenticate access to the maildrop. In
 order for this command to be successful, the POP3
 client must use a reserved TCP port (port < 1024) to
 connect tothe server. The POP3 server uses the
 argument pair from the USER and RPOP commands to
 determine if the client should be given access to
 the appropriate maildrop. Unlike the PASS command
 however, the POP3 server considers if the remote user
 specified by the RPOP command who resides on the POP3
 client host is allowed to access the maildrop for the
 user specified by the USER command (e.g., on Berkeley
 UNIX, the .rhosts mechanism is used). With the
 exception of this differing in authentication, this
 command is identical to the PASS command.
 Note that the use of this feature has allowed much wider
 penetration into numerous hosts on local networks (and
 sometimes remote networks) by those who gain illegal
 access to computers by guessing passwords or otherwise
 breaking into the system.
 Possible Responses:
 +OK maildrop locked and ready
 -ERR permission denied
 Examples:
 C: USER mrose
 S: +OK mrose is a real hoopy frood
 C: RPOP mrose
 S: +OK mrose's maildrop has 2 messages (320
 octets)
 Minimal POP3 Commands:
 USER name valid in the AUTHORIZATION state
 PASS string
 QUIT
 STAT valid in the TRANSACTION state
 LIST [msg]
 RETR msg
 DELE msg
 NOOP
 LAST
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 RSET
 QUIT valid in the UPDATE state
 Optional POP3 Commands:
 RPOP user valid in the AUTHORIZATION state
 TOP msg n valid in the TRANSACTION state
 POP3 Replies:
 +OK
 -ERR
 Note that with the exception of the STAT command, the reply given
 by the POP3 server to any command is significant only to "+OK"
 and "-ERR". Any text occurring after this reply may be ignored
 by the client.
Example POP3 Session
 S: <wait for connection on TCP port 110>
 ...
 C: <open connection>
 S: +OK dewey POP3 server ready (Comments to: PostMaster@UDEL.EDU)
 C: USER mrose
 S: +OK mrose is a real hoopy frood
 C: PASS secret
 S: +OK mrose's maildrop has 2 messages (320 octets)
 C: STAT
 S: +OK 2 320
 C: LIST
 S: +OK 2 messages (320 octets)
 S: 1 120
 S: 2 200
 S: .
 C: RETR 1
 S: +OK 120 octets
 S: <the POP3 server sends message 1>
 S: .
 C: DELE 1
 S: +OK message 1 deleted
 C: RETR 2
 S: +OK 200 octets
 S: <the POP3 server sends message 2>
 S: .
 C: DELE 2
 S: +OK message 2 deleted
 C: QUIT
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 S: +OK dewey POP3 server signing off (maildrop empty)
 C: <close connection>
 S: <wait for next connection>
Message Format
 All messages transmitted during a POP3 session are assumed to conform
 to the standard for the format of Internet text messages [RFC822].
 It is important to note that the byte count for a message on the
 server host may differ from the octet count assigned to that message
 due to local conventions for designating end-of-line. Usually,
 during the AUTHORIZATION state of the POP3 session, the POP3 client
 can calculate the size of each message in octets when it parses the
 maildrop into messages. For example, if the POP3 server host
 internally represents end-of-line as a single character, then the
 POP3 server simply counts each occurrence of this character in a
 message as two octets. Note that lines in the message which start
 with the termination octet need not be counted twice, since the POP3
 client will remove all byte-stuffed termination characters when it
 receives a multi-line response.
The POP and the Split-UA model
 The underlying paradigm in which the POP3 functions is that of a
 split-UA model. The POP3 client host, being a remote PC based
 workstation, acts solely as a client to the message transport system.
 It does not provide delivery/authentication services to others.
 Hence, it is acting as a UA, on behalf of the person using the
 workstation. Furthermore, the workstation uses SMTP to enter mail
 into the MTS.
 In this sense, we have two UA functions which interface to the
 message transport system: Posting (SMTP) and Retrieval (POP3). The
 entity which supports this type of environment is called a split-UA
 (since the user agent is split between two hosts which must
 interoperate to provide these functions).
 ASIDE: Others might term this a remote-UA instead.
 There are arguments supporting the use of both terms.
 This memo has explicitly referenced TCP as the underlying transport
 agent for the POP3. This need not be the case. In the MZnet split-
 UA, for example, personal micro-computer systems are used which do
 not have IP-style networking capability. To connect to the POP3
 server host, a PC establishes a terminal connection using some simple
 protocol (PhoneNet). A program on the PC drives the connection,
 first establishing a login session as a normal user. The login shell
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 for this pseudo-user is a program which drives the other half of the
 terminal protocol and communicates with one of two servers. Although
 MZnet can support several PCs, a single pseudo-user login is present
 on the server host. The user-id and password for this pseudo-user
 login is known to all members of MZnet. Hence, the first action of
 the login shell, after starting the terminal protocol, is to demand a
 USER/PASS authorization pair from the PC. This second level of
 authorization is used to ascertain who is interacting with the MTS.
 Although the server host is deemed to support a "trusted" MTS entity,
 PCs in MZnet are not. Naturally, the USER/PASS authorization pair
 for a PC is known only to the owner of the PC (in theory, at least).
 After successfully verifying the identity of the client, a modified
 SMTP server is started, and the PC posts mail with the server host.
 After the QUIT command is given to the SMTP server and it terminates,
 a modified POP3 server is started, and the PC retrieves mail from the
 server host. After the QUIT command is given to the POP3 server and
 it terminates, the login shell for the pseudo-user terminates the
 terminal protocol and logs the job out. The PC then closes the
 terminal connection to the server host.
 The SMTP server used by MZnet is modified in the sense that it knows
 that it's talking to a user agent and not a "trusted" entity in the
 message transport system. Hence, it does performs the validation
 activities normally performed by an entity in the MTS when it accepts
 a message from a UA.
 The POP3 server used by MZnet is modified in the sense that it does
 not require a USER/PASS combination before entering the TRANSACTION
 state. The reason for this (of course) is that the PC has already
 identified itself during the second-level authorization step
 described above.
 NOTE: Truth in advertising laws require that the author
 of this memo state that MZnet has not actually been
 fully implemented. The concepts presented and proven
 by the project led to the notion of the MZnet
 split-slot model. This notion has inspired the
 split-UA concept described in this memo, led to the
 author's interest in the POP, and heavily influenced
 the the description of the POP3 herein.
 In fact, some UAs present in the Internet already support the notion
 of posting directly to an SMTP server and retrieving mail directly
 from a POP server, even if the POP server and client resided on the
 same host!
 ASIDE: this discussion raises an issue which this memo
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 purposedly avoids: how does SMTP know that it's talking
 to a "trusted" MTS entity?
References
 [MZnet] Stefferud, E., J. Sweet, and T. Domae, "MZnet: Mail
 Service for Personal Micro-Computer Systems",
 Proceedings, IFIP 6.5 International Conference on
 Computer Message Systems, Nottingham, U.K., May 1984.
 [RFC821] Postel, J., "Simple Mail Transfer Protocol",
 USC/Information Sciences Institute, August 1982.
 [RFC822] Crocker, D., "Standard for the Format of ARPA-Internet
 Text Messages", University of Delaware, August 1982.
 [RFC937] Butler, M., J. Postel, D. Chase, J. Goldberger, and J.
 Reynolds, "Post Office Protocol - Version 2", RFC 937,
 USC/Information Sciences Institute, February 1985.
 [RFC1060] Reynolds, J., and J. Postel, "Assigned Numbers", RFC
 1060, USC/Information Sciences Institute, March 1990.
Security Considerations
 Security issues are not discussed in this memo.
Author's Address:
 Marshall T. Rose
 Performance Systems International
 5201 Great America Parkway
 Suite 3106
 Santa Clara, CA 95054
 Phone: +1 408 562 6222
 EMail: mrose@psi.com
 X.500: rose, psi, us
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