The IMAP ENABLE Extension
RFC 5161

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Network Working Group                                A. Gulbrandsen, Ed.
Request for Comments: 5161                        Oryx Mail Systems GmbH
Category: Standards Track                               A. Melnikov, Ed.
                                                           Isode Limited
                                                              March 2008

                       The IMAP ENABLE Extension

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.

Abstract

   Most IMAP extensions are used by the client when it wants to and the
   server supports it.  However, a few extensions require the server to
   know whether a client supports that extension.  The ENABLE extension
   allows an IMAP client to say which extensions it supports.

1.  Overview

   Several IMAP extensions allow the server to return unsolicited
   responses specific to these extensions in certain circumstances.
   However, servers cannot send those unsolicited responses until they
   know that the clients support such extensions and thus won't choke on
   the extension response data.

   Up until now, extensions have typically stated that a server cannot
   send the unsolicited responses until after the client has used a
   command with the extension data (i.e., at that point the server knows
   the client is aware of the extension).  CONDSTORE ([RFC4551]),
   ANNOTATE ([ANNOTATE]), and some extensions under consideration at the
   moment use various commands to enable server extensions.  For
   example, CONDSTORE uses a SELECT or FETCH parameter, and ANNOTATE
   uses a side effect of FETCH.

   The ENABLE extension provides an explicit indication from the client
   that it supports particular extensions.  This is done using a new
   ENABLE command.

   An IMAP server that supports ENABLE advertises this by including the
   word ENABLE in its capability list.

Gulbrandsen & Melnikov      Standards Track                     [Page 1]
RFC 5161               The IMAP ENABLE Extension              March 2008

   Most IMAP extensions do not require the client to enable the
   extension in any way.

2.  Conventions Used in This Document

   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 [RFC2119].

   Formal syntax is defined by [RFC5234] and [RFC3501].

   Example lines prefaced by "C:" are sent by the client and ones
   prefaced by "S:" by the server.  The five characters [...] means that
   something has been elided.

3.  Protocol Changes

3.1.  The ENABLE Command

   Arguments: capability names

   Result:    OK: Relevant capabilities enabled
              BAD: No arguments, or syntax error in an argument

   The ENABLE command takes a list of capability names, and requests the
   server to enable the named extensions.  Once enabled using ENABLE,
   each extension remains active until the IMAP connection is closed.
   For each argument, the server does the following:

   - If the argument is not an extension known to the server, the server
     MUST ignore the argument.

   - If the argument is an extension known to the server, and it is not
     specifically permitted to be enabled using ENABLE, the server MUST
     ignore the argument.  (Note that knowing about an extension doesn't
     necessarily imply supporting that extension.)

   - If the argument is an extension that is supported by the server and
     that needs to be enabled, the server MUST enable the extension for
     the duration of the connection.  At present, this applies only to
     CONDSTORE ([RFC4551]).  Note that once an extension is enabled,
     there is no way to disable it.

   If the ENABLE command is successful, the server MUST send an untagged
   ENABLED response (see Section 3.2).

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RFC 5161               The IMAP ENABLE Extension              March 2008

   Clients SHOULD only include extensions that need to be enabled by the
   server.  At the time of publication, CONDSTORE is the only such
   extension (i.e., ENABLE CONDSTORE is an additional "CONDSTORE
   enabling command" as defined in [RFC4551]).  Future RFCs may add to
   this list.

   The ENABLE command is only valid in the authenticated state (see
   [RFC3501]), before any mailbox is selected.  Clients MUST NOT issue
   ENABLE once they SELECT/EXAMINE a mailbox; however, server
   implementations don't have to check that no mailbox is selected or
   was previously selected during the duration of a connection.

   The ENABLE command can be issued multiple times in a session.  It is