POP3 Extension Mechanism
RFC 2449

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Network Working Group                                         R. Gellens
Request for Comments: 2449                                      Qualcomm
Updates: 1939                                                  C. Newman
Category: Standards Track                                       Innosoft
                                                            L. Lundblade
                                                                Qualcomm
                                                           November 1998

                        POP3 Extension Mechanism

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.

Copyright Notice

   Copyright (C) The Internet Society (1998).  All Rights Reserved.

IESG Note

   This extension to the POP3 protocol is to be used by a server to
   express policy descisions taken by the server administrator.  It is
   not an endorsement of implementations of further POP3 extensions
   generally.  It is the general view that the POP3 protocol should stay
   simple, and for the simple purpose of downloading email from a mail
   server.  If more complicated operations are needed, the IMAP protocol
   [RFC 2060] should be used.  The first paragraph of section 7 should
   be read very carefully.

Table of Contents

    1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . .  2
    2.  Conventions Used in this Document . . . . . . . . . . . . .   3
    3.  General Command and Response Grammar . . . . . . . . . . . .  3
    4.  Parameter and Response Lengths  . . . . . . . . . . . . . .   4
    5.  The CAPA Command . . . . . . . . . . . . . . . . . . . . . .  4
    6.  Initial Set of Capabilities . . . . . . . . . . . . . . . .   5
      6.1.  TOP capability . . . . . . . . . . . . . . . . . . . . .  6
      6.2.  USER capability . . . . . . . . . . . . . . . . . . . .   6
      6.3.  SASL capability  . . . . . . . . . . . . . . . . . . . .  7
      6.4.  RESP-CODES capability . . . . . . . . . . . . . . . . .   8
      6.5.  LOGIN-DELAY capability . . . . . . . . . . . . . . . . .  8
      6.6.  PIPELINING capability . . . . . . . . . . . . . . . . .   9

Gellens, et. al.            Standards Track                     [Page 1]
RFC 2449                POP3 Extension Mechanism           November 1998

      6.7.  EXPIRE capability  . . . . . . . . . . . . . . . . . . . 10
      6.8.  UIDL capability . . . . . . . . . . . . . . . . . . . .  13
      6.9.  IMPLEMENTATION capability  . . . . . . . . . . . . . . . 13
    7.  Future Extensions to POP3 . . . . . . . . . . . . . . . . .  14
    8.  Extended POP3 Response Codes . . . . . . . . . . . . . . . . 14
      8.1.  Initial POP3 response codes . . . . . . . . . . . . . .  15
        8.1.1.  The LOGIN-DELAY response code  . . . . . . . . . . . 15
        8.1.2.  The IN-USE response code  . . . . . . . . . . . . .  16
    9.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . 16
   10.  Security Considerations . . . . . . . . . . . . . . . . . .  17
   11.  Acknowledgments  . . . . . . . . . . . . . . . . . . . . . . 17
   12.  References  . . . . . . . . . . . . . . . . . . . . . . . .  17
   13.  Authors' Addresses  . . . . . . . . . . . . . . . . . . . .  18
   14.  Full Copyright Statement . . . . . . . . . . . . . . . . . . 19

1.  Introduction

   The Post Office Protocol version 3 [POP3] is very widely used.
   However, while it includes some optional commands (and some useful
   protocol extensions have been published), it lacks a mechanism for
   advertising support for these extensions or for behavior variations.

   Currently these optional features and extensions can only be detected
   by probing, if at all.  This is at best inefficient, and possibly
   worse.  As a result, some clients have manual configuration options
   for POP3 server capabilities.

   Because one of the most important characteristics of POP3 is its
   simplicity, it is desirable that extensions be few in number (see
   section 7).  However, some extensions are necessary (such as ones
   that provide improved security [POP-AUTH]), while others are very
   desirable in certain situations.  In addition, a means for
   discovering server behavior is needed.

   This memo updates RFC 1939 [POP3] to define a mechanism to announce
   support for optional commands, extensions, and unconditional server
   behavior.  Included is an initial set of currently deployed
   capabilities which vary between server implementations, and several
   new capabilities (SASL, RESP-CODES, LOGIN-DELAY, PIPELINING, EXPIRE
   and IMPLEMENTATION).  This document also extends POP3 error messages
   so that machine parsable codes can be provided to the client.  An