Interactive Mail Access Protocol: Version 2
RFC 1176

Document Type RFC - Experimental (August 1990; No errata)
Obsoletes RFC 1064
Author Mark Crispin 
Last updated 2013-03-02
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Network Working Group                                         M. Crispin
Request for Comments: 1176                                    Washington
Obsoletes: RFC 1064                                          August 1990


Status of this Memo

   This RFC suggests a method for personal computers and workstations to
   dynamically access mail from a mailbox server ("repository").  It
   obosoletes RFC 1064.  This RFC specifies an Experimental Protocol for
   the Internet community.  Discussion and suggestions for improvement
   are requested.  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.


   The intent of the Interactive Mail Access Protocol, Version 2 (IMAP2)
   is to allow a workstation, personal computer, or similar small
   machine to access electronic mail from a mailbox server.  Since the
   distinction between personal computers and workstations is blurring
   over time, it is desirable to have a single solution that addresses
   the need in a general fashion.  IMAP2 is the "glue" of a distributed
   electronic mail system consisting of a family of client and server
   implementations on a wide variety of platforms, from small single-
   tasking personal computing engines to complex multi-user timesharing

   Although different in many ways from the Post Office Protocols (POP2
   and POP3, hereafter referred to collectively as "POP") described in
   RFC 937 and RFC 1081, IMAP2 may be thought of as a functional
   superset of these.  RFC 937 was used as a model for this RFC.  There
   was a cognizant reason for this; POP deals with a similar problem,
   albeit with a less comprehensive solution, and it was desirable to
   offer a basis for comparison.

   Like POP, IMAP2 specifies a means of accessing stored mail and not of
   posting mail; this function is handled by a mail transfer protocol
   such as SMTP (RFC 821).

   This protocol assumes a reliable data stream such as provided by TCP
   or any similar protocol.  When TCP is used, the IMAP2 server listens
   on port 143.

Crispin                                                         [Page 1]
RFC 1176                         IMAP2                       August 1990

System Model and Philosophy

   Electronic mail is a primary means of communication for the widely
   spread Internet community.  The advent of distributed personal
   computers and workstations has forced a significant rethinking of the
   mechanisms employed to manage electronic mail.  With mainframes, each
   user tends to receive and process mail at the computer he uses most
   of the time, his "primary host".  The first inclination of many users
   when an independent workstation is placed in front of them is to
   begin receiving mail at the workstation, and many vendors have
   implemented facilities to do this.  However, this approach has
   several disadvantages:

      (1) Personal computers and many workstations have a software
      design that gives full control of all aspects of the system to the
      user at the console.  As a result, background tasks such as
      receiving mail may not run for long periods of time; either
      because the user is asking to use all the machine's resources, or
      because the user has (perhaps accidentally) manipulated the
      environment in such a way that it prevents mail reception.  In
      many personal computers, the operating system is single-tasking
      and this is the only mode of operation.  Any of these conditions
      could lead to repeated failed delivery attempts by outside agents.

      (2) The hardware failure of a single machine can keep its user
      "off the air" for a considerable time, since repair of individual
      units may be delayed.  Given the growing number of personal
      computers and workstations spread throughout office environments,
      quick repair of such systems is not assured.  On the other hand, a
      central mainframe is generally repaired soon after failure.

      (3) Personal computers and workstations are often not backed up
      with as much diligence as a central mainframe, if at all.

      (4) It is more difficult to keep track of mailing addresses when
      each person is associated with a distinct machine.  Consider the
      difficulty in keeping track of many postal addresses or phone
      numbers, particularly if there was no single address or phone
      number for an organization through which you could reach any
      person in that organization.  Traditionally, electronic mail on
      the ARPANET involved remembering a name and one of several "hosts"
      (machines) whose name reflected the organization in which the
      individual worked.  This was suitable at a time when most
      organizations had only one central host.  It is less satisfactory
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