Post Office Protocol: Version 2
RFC 937

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Network Working Group                                          M. Butler
Request for Comments: 937                                      J. Postel
                                                                D. Chase
                                                           J. Goldberger
                                                          J. K. Reynolds
Obsoletes: RFC 918                                                   ISI
                                                           February 1985

                    POST OFFICE PROTOCOL - VERSION 2

Status of this Memo

   This RFC suggests a simple method for workstations to dynamically
   access mail from a mailbox server.  This RFC specifies a proposed
   protocol for the ARPA-Internet community, and requests discussion and
   suggestions for improvement.  This memo is a revision of RFC 918.
   Distribution of this memo is unlimited.

Introduction

   The intent of the Post Office Protocol Version 2 (POP2) is to allow a
   user's workstation to access mail from a mailbox server.  It is
   expected that mail will be posted from the workstation to the mailbox
   server via the Simple Mail Transfer Protocol (SMTP).  For further
   information see RFC-821 [1] and RFC-822 [2].

   This protocol assumes a reliable data stream such as provided by TCP
   or any similar protocol.  When TCP is used, the POP2 server listens
   on port 109 [4].

System Model and Philosophy

   While we view the workstation as an Internet host in the sense that
   it implements IP, we do not expect the workstation to contain the
   user's mailbox.  We expect the mailbox to be on a server machine.

   We believe it is important for the mailbox to be on an "always up"
   machine and that a workstation may be frequently powered down, or
   otherwise unavailable as an SMTP server.

   POP2 is designed for an environment of workstations and servers on a
   low-delay, high-throughput, local networks (such as Ethernets).  POP2
   may be useful in other environments as well, but if the environment
   is substantially different, a different division of labor between the
   client and server may be appropriate, and a different protocol
   required.

   Suppose the user's real name is John Smith, the user's machine is
   called FIDO, and that the mailbox server is called DOG-HOUSE.  Then

Butler, et. al.                                                 [Page 1]



RFC 937                                                    February 1985
Post Office Protocol

   we expect the user's mail to be addressed to JSmith@DOG-HOUSE.ARPA
   (not JSmith@FIDO.ARPA).

   That is, the destination of the mail is the mailbox on the server
   machine.  The POP2 protocol and the workstation are merely a
   mechanism for viewing the messages in the mailbox.

   The user is not tied to any particular workstation for accessing his
   mail.  The workstation does not appear as any part of the mailbox
   address.

   This is a very simple protocol.  This is not a user interface.  We
   expect that there is a program in the workstation that is friendly to
   the user.  This protocol is not "user friendly".  One basic rule of
   this protocol is "if anything goes wrong close the connection".
   Another basic rule is to have few options.

   POP2 does not parse messages in any way.  It does not analyze message
   headers (Date:, From:, To:, Cc:, or Subject:).  POP2 simply transmits
   whole messages from a mailbox server to a client workstation.

The Protocol

   The POP2 protocol is a sequence of commands and replies.  The design
   draws from many previous protocols of the ARPA-Internet community.

      The server must be listening for a connection.  When a connection
      is opened the server sends a greeting message and waits for
      commands.  When commands are received the server acts on them and
      responds with replies.

      The client opens a connection, waits for the greeting, then sends
      the HELO command with the user name and password arguments to
      establish authorization to access mailboxes.  The server returns
      the number of messages in the default mailbox.

      The client may read the default mailbox associated with the user
      name or may select another mailbox by using the FOLD command.  The
      server returns the number of messages in the mailbox selected.

      The client begins a message reading transaction with a READ
      command.  The read command may optionally indicate which message
      number to read, the default is the current message (incremented
      when a message is read and set to one when a new folder is
      selected).  The server returns the number of characters in the
      message.

Butler, et. al.                                                 [Page 2]



RFC 937                                                    February 1985
Post Office Protocol

      The client asks for the content of the message to be sent with the
      RETR command.  The server sends the message data.