Post Office Protocol - Version 3
RFC 1460

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Network Working Group                                            M. Rose
Request for Comments: 1460                  Dover Beach Consulting, Inc.
Obsoletes: 1225                                                June 1993

                    Post Office Protocol - Version 3

Status of this Memo

   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 revision to RFC 1225, a Draft Standard.  It makes the
   following changes from that document:

        - the RPOP facility is removed;

        - the optional APOP facility is added (which is in interoperable,
          operational use in at least three implementations);

        - a typo was corrected with respect to the interaction of LAST
          and RSET;

        - section numbers were added; and,

        - an acknowledgements section was added.

1. 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 [RFC821] 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

Rose                                                            [Page 1]
RFC 1460                          POP3                         June 1993

   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
   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.

2. 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.

3. Basic Operation

   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

Rose                                                            [Page 2]
RFC 1460                          POP3                         June 1993

   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.

   Hence a multi-line response is terminated with the five octets
   "CRLF.CRLF".  When examining a multi-line response, the client checks