Post Office Protocol - Version 3
RFC 1939

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Network Working Group                                           J. Myers
Request for Comments: 1939                               Carnegie Mellon
STD: 53                                                          M. Rose
Obsoletes: 1725                             Dover Beach Consulting, Inc.
Category: Standards Track                                       May 1996

                    Post Office Protocol - Version 3

Status of this Memo

   This document specifies an Internet standards track protocol for the
   Internet community, and requests discussion and suggestions for
   improvements.  Please refer to the current edition of the "Internet
   Official Protocol Standards" (STD 1) for the standardization state
   and status of this protocol.  Distribution of this memo is unlimited.

Table of Contents

   1. Introduction ................................................    2
   2. A Short Digression ..........................................    2
   3. Basic Operation .............................................    3
   4. The AUTHORIZATION State .....................................    4
      QUIT Command ................................................    5
   5. The TRANSACTION State .......................................    5
      STAT Command ................................................    6
      LIST Command ................................................    6
      RETR Command ................................................    8
      DELE Command ................................................    8
      NOOP Command ................................................    9
      RSET Command ................................................    9
   6. The UPDATE State ............................................   10
      QUIT Command ................................................   10
   7. Optional POP3 Commands ......................................   11
      TOP Command .................................................   11
      UIDL Command ................................................   12
      USER Command ................................................   13
      PASS Command ................................................   14
      APOP Command ................................................   15
   8. Scaling and Operational Considerations ......................   16
   9. POP3 Command Summary ........................................   18
   10. Example POP3 Session .......................................   19
   11. Message Format .............................................   19
   12. References .................................................   20
   13. Security Considerations ....................................   20
   14. Acknowledgements ...........................................   20
   15. Authors' Addresses .........................................   21
   Appendix A. Differences from RFC 1725 ..........................   22

Myers & Rose                Standards Track                     [Page 1]
RFC 1939                          POP3                          May 1996

   Appendix B. Command Index ......................................   23

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
   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 protocol
   is used to allow a workstation to retrieve mail that the server is
   holding for it.

   POP3 is not intended to provide extensive manipulation operations of
   mail on the server; normally, mail is downloaded and then deleted.  A
   more advanced (and complex) protocol, IMAP4, is discussed in
   [RFC1730].

   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