IMAP4 Multi-Accessed Mailbox Practice
RFC 2180

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Network Working Group                                          M. Gahrns
Request for Comments: 2180                                     Microsoft
Category: Informational                                        July 1997

                 IMAP4 Multi-Accessed Mailbox Practice

Status of this Memo

   This memo provides information for the Internet community.  This memo
   does not specify an Internet standard of any kind.  Distribution of
   this memo is unlimited.

1. Abstract

   IMAP4[RFC-2060] is rich client/server protocol that allows a client
   to access and manipulate electronic mail messages on a server.
   Within the protocol framework, it is possible to have differing
   results for particular client/server interactions. If a protocol does
   not allow for this, it is often unduly restrictive.

   For example, when multiple clients are accessing a mailbox and one
   attempts to delete the mailbox, an IMAP4 server may choose to
   implement a solution based upon server architectural constraints or
   individual preference.

   With this flexibility comes greater client responsibility.  It is not
   sufficient for a client to be written based upon the behavior of a
   particular IMAP server.  Rather the client must be based upon the
   behavior allowed by the protocol.

   By documenting common IMAP4 server practice for the case of
   simultaneous client access to a mailbox, we hope to ensure the widest
   amount of inter-operation between IMAP4 clients and servers.

   The behavior described in this document reflects the practice of some
   existing servers or behavior that the consensus of the IMAP mailing
   list has deemed to be reasonable.  The behavior described within this
   document is believed to be [RFC-2060] compliant. However, this
   document is not meant to define IMAP4 compliance, nor is it an
   exhaustive list of valid IMAP4 behavior. [RFC-2060] must always be
   consulted to determine IMAP4 compliance, especially for server
   behavior not described within this document.

Gahrns                       Informational                      [Page 1]
RFC 2180         IMAP4 Multi-Accessed Mailbox Practice         July 1997

2. Conventions used in this document

   In examples,"C1:", "C2:" and "C3:" indicate lines sent by 3 different
   clients (client #1, client #2 and client #3) that are connected to a
   server.  "S1:", "S2:" and "S3:" indicated lines sent by the server to
   client #1, client #2 and client #3 respectively.

   A shared mailbox, is a mailbox that can be used by multiple users.

   A multi-accessed mailbox, is a mailbox that has multiple clients
   simultaneously accessing it.

   A client is said to have accessed a mailbox after a successful SELECT
   or EXAMINE command.

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED",  "MAY", and "OPTIONAL" in this
   document are to be interpreted as described in [RFC-2119].

3. Deletion/Renaming of a multi-accessed mailbox

   If an external agent or multiple clients are accessing a mailbox,
   care must be taken when handling the deletion or renaming of the
   mailbox. Following are some strategies an IMAP server may choose to
   use when dealing with this situation.

3.1. The server MAY fail the DELETE/RENAME command of a multi-accessed
     mailbox

   In some cases, this behavior may not be practical.  For example, if a
   large number of clients are accessing a shared mailbox, the window in
   which no clients have the mailbox accessed may be small or non-
   existent, effectively rendering the mailbox undeletable or
   unrenamable.

   Example:

   <Client #1 and Client #2 have mailbox FOO accessed. Client #1 tries
   to DELETE the mailbox and is refused>

             C1: A001 DELETE FOO
             S1: A001 NO Mailbox FOO is in use by another user.

Gahrns                       Informational                      [Page 2]
RFC 2180         IMAP4 Multi-Accessed Mailbox Practice         July 1997

3.2. The server MAY allow the DELETE command of a multi-accessed
     mailbox, but keep the information in the mailbox available for
     those clients that currently have access to the mailbox.

   When all clients have finished accessing the mailbox, it is
   permanently removed.  For clients that do not already have access to
   the mailbox, the 'ghosted' mailbox would not be available.  For
   example, it would not be returned to these clients in a subsequent
   LIST or LSUB command and would not be a valid mailbox argument to any
   other IMAP command until the reference count of clients accessing the
   mailbox reached 0.

   In some cases, this behavior may not be desirable. For example if
   someone created a mailbox with offensive or sensitive information,
   one might prefer to have the mailbox deleted and all access to the
   information contained within removed immediately, rather than
   continuing to allow access until the client closes the mailbox.