path: root/doc/rfc/rfc1225.txt

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



Rose                                                            [Page 1]
^L
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.



Rose                                                            [Page 2]
^L
RFC 1225                          POP3                          May 1991


   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),



Rose                                                            [Page 3]
^L
RFC 1225                          POP3                          May 1991


   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)



Rose                                                            [Page 4]
^L
RFC 1225                          POP3                          May 1991


               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.



Rose                                                            [Page 5]
^L
RFC 1225                          POP3                          May 1991


                 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



Rose                                                            [Page 6]
^L
RFC 1225                          POP3                          May 1991


                 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



Rose                                                            [Page 7]
^L
RFC 1225                          POP3                          May 1991


                 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:



Rose                                                            [Page 8]
^L
RFC 1225                          POP3                          May 1991


                   +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:




Rose                                                            [Page 9]
^L
RFC 1225                          POP3                          May 1991


                 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



Rose                                                           [Page 10]
^L
RFC 1225                          POP3                          May 1991


   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



Rose                                                           [Page 11]
^L
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



Rose                                                           [Page 12]
^L
RFC 1225                          POP3                          May 1991


           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



Rose                                                           [Page 13]
^L
RFC 1225                          POP3                          May 1991


    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



Rose                                                           [Page 14]
^L
RFC 1225                          POP3                          May 1991


   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



Rose                                                           [Page 15]
^L
RFC 1225                          POP3                          May 1991


                 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












Rose                                                           [Page 16]
^L