This is a purely informative rendering of an RFC that includes verified errata. This rendering may not be used as a reference.

The following 'Verified' errata have been incorporated in this document: EID 2002, EID 4046
Network Working Group                                         P. Resnick
Request for Comments: 4469                         QUALCOMM Incorporated
Updates: 3501, 3502                                           April 2006
Category: Standards Track

       Internet Message Access Protocol (IMAP) CATENATE 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.

Copyright Notice

   Copyright (C) The Internet Society (2006).


   The CATENATE extension to the Internet Message Access Protocol (IMAP)
   extends the APPEND command to allow clients to create messages on the
   IMAP server that may contain a combination of new data along with
   parts of (or entire) messages already on the server.  Using this
   extension, the client can catenate parts of an already existing
   message onto a new message without having to first download the data
   and then upload it back to the server.

1.  Introduction

   The CATENATE extension to the Internet Message Access Protocol (IMAP)
   [1] allows the client to create a message on the server that can
   include the text of messages (or parts of messages) that already
   exist on the server without having to FETCH them and APPEND them back
   to the server.  The CATENATE extension extends the APPEND command so
   that, instead of a single message literal, the command can take as
   arguments any combination of message literals (as described in IMAP
   [1]) and message URLs (as described in the IMAP URL Scheme [2]
   specification).  The server takes all the pieces and catenates them
   into the output message.  The CATENATE extension can also coexist
   with the MULTIAPPEND extension [3] to APPEND multiple messages in a
   single command.

   There are some obvious uses for the CATENATE extension.  The
   motivating use case was to provide a way for a resource-constrained
   client to compose a message for subsequent submission that contains
   data that already exists in that client's IMAP store.  Because the
   client does not have to download and re-upload potentially large
   message parts, bandwidth and processing limitations do not have as
   much impact.  In addition, since the client can create a message in
   its own IMAP store, the command also addresses the desire of the
   client to archive a copy of a sent message without having to upload
   the message twice.  (Mechanisms for sending the message are outside
   the scope of this document.)

   The extended APPEND command can also be used to copy parts of a
   message to another mailbox for archival purposes while getting rid of
   undesired parts.  In environments where server storage is limited, a
   client could get rid of large message parts by copying over only the
   necessary parts and then deleting the original message.  The
   mechanism could also be used to add data to a message (such as
   prepending message header fields) or to include other data by making
   a copy of the original and catenating the new data.

2.  The CATENATE Capability

   A server that supports this extension returns "CATENATE" as one of
   the responses to the CAPABILITY command.

3.  The APPEND Command

   Arguments:  mailbox name
               (The following can be repeated in the presence of the
               MULTIAPPEND extension [3])
               OPTIONAL flag parenthesized list
               OPTIONAL date/time string
               a single message literal or one or more message parts to
               catenate, specified as:
                           message literal
                           message (or message part) URL

   Responses:  OPTIONAL NO responses: BADURL, TOOBIG

   Result:     OK -  append completed
               NO -  append error: can't append to that mailbox, error
                     in flags or date/time or message text, or can't
                     fetch that data
               BAD - command unknown or arguments invalid

   The APPEND command concatenates all the message parts and appends
   them as a new message to the end of the specified mailbox.  The
   parenthesized flag list and date/time string set the flags and the
   internal date, respectively, as described in IMAP [1].  The
   subsequent command parameters specify the message parts that are
   appended sequentially to the output message.

   If the original form of APPEND is used, a message literal follows the
   optional flag list and date/time string, which is appended as
   described in IMAP [1].  If the extended form is used, "CATENATE" and
   a parenthesized list of message literals and message URLs follows,
   each of which is appended to the new message.  If a message literal
   is specified (indicated by "TEXT"), the octets following the count
   are appended.  If a message URL is specified (indicated by "URL"),
   the octets of the body part pointed to by that URL are appended, as
   if the literal returned in a FETCH BODY response were put in place of
   the message part specifier.  The APPEND command does not cause the
   \Seen flag to be set for any catenated body part.  The APPEND command
   does not change the selected mailbox.

   In the extended APPEND command, the string following "URL" is an IMAP
   URL [2] and is interpreted according to the rules of [2].  The
   present document only describes the behavior of the command using
   IMAP URLs that refer to specific messages or message parts on the
   current IMAP server from the current authenticated IMAP session.
   Because of that, only relative IMAP message or message part URLs
   (i.e., those having no scheme or <iserver>) are used.  The base URL

   for evaluating the relative URL is considered "imap://user@server/",
   where "user" is the user name of the currently authenticated user and
   "server" is the domain name of the current server.  When in the
   selected state, the base URL is considered
   "imap://user@server/mailbox", where "mailbox" is the encoded name of
   the currently selected mailbox.  Additionally, since the APPEND
   command is valid in the authenticated state of an IMAP session, no
   further LOGIN or AUTHENTICATE command is performed for URLs specified
   in the extended APPEND command.

      Note: Use of an absolute IMAP URL or any URL that refers to
      anything other than a message or message part from the current
      authenticated IMAP session is outside the scope of this document
      and would require an extension to this specification, and a server
      implementing only this specification would return NO to such a

   The client is responsible for making sure that the catenated message
   is in the format of an Internet Message Format (RFC 2822) [4] or
   Multipurpose Internet Mail Extension (MIME) [5] message.  In
   particular, when a URL is catenated, the server copies octets,
   unchanged, from the indicated message or message part to the
   catenated message.  It does no data conversion (e.g., MIME transfer
   encodings) nor any verification that the data is appropriate for the
   MIME part of the message into which it is inserted.  The client is
   also responsible for inserting appropriate MIME boundaries between
   body parts, and writing MIME Content-Type and Content-Transfer-
   Encoding lines as needed in the appropriate places.

   Responses behave just as the original APPEND command described in
   IMAP [1].  If the server implements the IMAP UIDPLUS extension [6],
   it will also return an APPENDUID response code in the tagged OK
   response.  Two response codes are provided in Section 4 that can be
   used in the tagged NO response if the APPEND command fails.

4.  Response Codes

   When a APPEND command fails, it may return a response code that
   describes a reason for the failure.

4.1.  BADURL Response

   The BADURL response code is returned if the APPEND fails to process
   one of the specified URLs.  Possible reasons for this are bad URL
   syntax, unrecognized URL schema, invalid message UID, or invalid body
   part.  The BADURL response code contains the first URL specified as a
   parameter to the APPEND command that has caused the operation to

4.2.  TOOBIG Response

   The TOOBIG response code is returned if the resulting message will
   exceed the 4-GB IMAP message limit.  This might happen, for example,
   if the client specifies 3 URLs for 2-GB messages.  Note that even if
   the server doesn't return TOOBIG, it still has to be defensive
   against misbehaving or malicious clients that try to construct a
   message over the 4-GB limit.  The server may also wish to return the
   TOOBIG response code if the resulting message exceeds a server-
   specific message size limit.

5.  Formal Syntax

   The following syntax specification uses the Augmented Backus-Naur
   Form (ABNF) [7] notation.  Elements not defined here can be found in
   the formal syntax of the ABNF [7], IMAP [1], and IMAP ABNF extensions
   [8] specifications.  Note that capability and resp-text-code are
   extended from the IMAP [1] specification and append-data is extended
   from the IMAP ABNF extensions [8] specification.

   append-data =/ "CATENATE" SP "(" cat-part *(SP cat-part) ")"

   cat-part = text-literal / url

   text-literal = "TEXT" SP literal

   url = "URL" SP astring

   resp-text-code =/ toobig-response-code / badurl-response-code

   toobig-response-code = "TOOBIG"

   badurl-response-code = "BADURL" SP url-resp-text

      url-resp-text = 1*(%x01-09 / 
                      %x0B-0C /
                      %x0E-5C /
                      %x5E-FE) ; Any TEXT-CHAR except "]"
EID 2002 (Verified) is as follows:

Section: 5

Original Text:

   url-resp-text = 1*(%x01-09 /
                      %x0B-0C /
                      %x0E-5B /
                      %x5D-FE) ; Any TEXT-CHAR except "]"

Corrected Text:

   url-resp-text = 1*(%x01-09 /
                      %x0B-0C /
                      %x0E-5C /
                      %x5E-FE) ; Any TEXT-CHAR except "]"
The skipped character %x5C is "\" not "]". I think the intent is to omit "]" since url-resp-text is used exclusively inside a [BADURL url-resp-text] response code, and they want to avoid aliasing the closing "]".
capability =/ "CATENATE" The astring in the definition of url and the url-resp-text in the definition of badurl-response-code each contain an imapurl as defined by [2]. 6. Acknowledgements Thanks to the members of the LEMONADE working group for their input. Special thanks to Alexey Melnikov for the examples. 7. Security Considerations The CATENATE extension does not raise any security considerations that are not present for the base protocol or in the use of IMAP URLs, and these issues are discussed in the IMAP [1] and IMAP URL [2] documents. 8. IANA Considerations IMAP4 capabilities are registered by publishing a standards track or IESG approved experimental RFC. The registry is currently located at <>. This document defines the CATENATE IMAP capability. The IANA has added this capability to the registry. Appendix A. Examples Lines not starting with "C: " or "S: " are continuations of the previous lines. The original message in examples 1 and 2 below (UID = 20) has the following structure: multipart/mixed MIME message with two body parts: 1. text/plain 2. application/x-zip-compressed Example 1: The following example demonstrates how a CATENATE client can replace an attachment in a draft message, without the need to download it to the client and upload it back. C: A003 APPEND Drafts (\Seen \Draft $MDNSent) CATENATE (URL "/Drafts;UIDVALIDITY=385759045/;UID=20/;section=HEADER" TEXT {42} S: + Ready for literal data C: C: --------------030308070208000400050907 C: URL "/Drafts;UIDVALIDITY=385759045/;UID=20/;section=1.MIME" URL "/Drafts;UIDVALIDITY=385759045/;UID=20/;section=1" TEXT {42} S: + Ready for literal data C: C: --------------030308070208000400050907 C: URL "/Drafts;UIDVALIDITY=385759045/;UID=30" TEXT {44} S: + Ready for literal data C: C: --------------030308070208000400050907-- C: ) S: A003 OK catenate append completed Example 2: The following example demonstrates how the CATENATE extension can be used to replace edited text in a draft message, as well as header fields for the top level message part (e.g., Subject has changed). The previous version of the draft is marked as \Deleted. Note that the server also supports the UIDPLUS extension, so the APPENDUID response code is returned in the successful OK response to the APPEND command. C: A003 APPEND Drafts (\Seen \Draft $MDNSent) CATENATE (TEXT {738} S: + Ready for literal data C: Return-Path: <> C: Received: from [] C: by via TCP (internal) with ESMTPA; C: Thu, 11 Nov 2004 16:57:07 +0000 C: Message-ID: <> C: Date: Thu, 12 Nov 2004 16:57:05 +0000 C: From: Bob Ar <> C: X-Accept-Language: en-us, en C: MIME-Version: 1.0 C: To: C: Subject: About our holiday trip C: Content-Type: multipart/mixed; C: boundary="------------030308070208000400050907" C: C: --------------030308070208000400050907 C: Content-Type: text/plain; charset=us-ascii; format=flowed C: Content-Transfer-Encoding: 7bit C: C: Our travel agent has sent the updated schedule. C: C: Cheers, C: Bob C: --------------030308070208000400050907 C: URL "/Drafts;UIDVALIDITY=385759045/;UID=20/;Section=2.MIME" URL "/Drafts;UIDVALIDITY=385759045/;UID=20/;Section=2" TEXT {44} S: + Ready for literal data C: C: --------------030308070208000400050907-- C: ) S: A003 OK [APPENDUID 385759045 45] append Completed C: A004 UID STORE 20 +FLAGS.SILENT (\Deleted) S: A004 OK STORE completed Example 3: The following example demonstrates how the CATENATE extension can be used to strip attachments. Below, a PowerPoint attachment was replaced by a small text part explaining that the attachment was stripped. C: A003 APPEND Drafts (\Seen \Draft $MDNSent) CATENATE (URL "/Drafts;UIDVALIDITY=385759045/;UID=21/;section=HEADER" TEXT {42} S: + Ready for literal data C: C: --------------030308070208000400050903 C: URL "/Drafts;UIDVALIDITY=385759045/;UID=21/;section=1.MIME" URL "/Drafts;UIDVALIDITY=385759045/;UID=21/;section=1" TEXT {255} S: + Ready for literal data C: C: --------------030308070208000400050903 C: Content-type: text/plain; charset="us-ascii" C: Content-transfer-encoding: 7bit C: C: This body part contained a Power Point presentation that was C: deleted upon your request. C: --------------030308070208000400050903-- C: ) S: A003 OK append Completed Example 4: The following example demonstrates a failed APPEND command. The server returns the BADURL response code to indicate that one of the provided URLs is invalid. This example also demonstrates how the CATENATE extension can be used to construct a digest of several messages. C: A003 APPEND Sent (\Seen $MDNSent) CATENATE (TEXT {541} S: + Ready for literal data C: Return-Path: <> C: Received: from [] C: by via TCP (internal) with ESMTPA; C: Thu, 11 Nov 2004 16:57:07 +0000 C: Message-ID: <> C: Date: Thu, 21 Nov 2004 16:57:05 +0000 C: From: Farren Oo <> C: X-Accept-Language: en-us, en C: MIME-Version: 1.0 C: To: C: Subject: Digest of the mailing list for today C: Content-Type: multipart/digest; C: boundary="------------030308070208000400050904" C: C: --------------030308070208000400050904 C: URL "/INBOX;UIDVALIDITY=785799047/;UID=11467" TEXT {42} S: + Ready for literal data C: C: --------------030308070208000400050904 C: URL "/INBOX;UIDVALIDITY=785799047/;UID=113330/;section=1.5.9" TEXT {42} S: + Ready for literal data C: C: --------------030308070208000400050904 C: URL "/INBOX;UIDVALIDITY=785799047/;UID=11916" TEXT {44} S: + Ready for literal data C: C: --------------030308070208000400050904-- C: ) S: A003 NO [BADURL /INBOX;UIDVALIDITY=785799047/;UID=113330; section=1.5.9] CATENATE append has failed, one message expunged
EID 4046 (Verified) is as follows:

Section: Appendix A

Original Text:

   S: A003 NO [BADURL "/INBOX;UIDVALIDITY=785799047/;UID=113330;
   section=1.5.9"] CATENATE append has failed, one message expunged

Corrected Text:

   S: A003 NO [BADURL /INBOX;UIDVALIDITY=785799047/;UID=113330;
   section=1.5.9] CATENATE append has failed, one message expunged
This example treats the url-resp-text in the badurl-response-code as though it were an astring. It is not: it is a bare imapurl, as stated in section 5:

"The astring in the definition of url and the url-resp-text in the
definition of badurl-response-code each contain an imapurl as defined
by [2]."

The example is incorrect, in that the double quotes should be removed so the url-resp-text is a valid imapurl, and only that.
Note that the server could have validated the URLs as they were received and therefore could have returned the tagged NO response with BADURL response-code in place of any continuation request after the URL was received. 9. Normative References [1] Crispin, M., "INTERNET MESSAGE ACCESS PROTOCOL - VERSION 4rev1", RFC 3501, March 2003. [2] Newman, C., "IMAP URL Scheme", RFC 2192, September 1997. [3] Crispin, M., "Internet Message Access Protocol (IMAP) - MULTIAPPEND Extension", RFC 3502, March 2003. [4] Resnick, P., "Internet Message Format", RFC 2822, April 2001. [5] Freed, N. and N. Borenstein, "Multipurpose Internet Mail Extensions (MIME) Part One: Format of Internet Message Bodies", RFC 2045, November 1996. [6] Crispin, M., "Internet Message Access Protocol (IMAP) - UIDPLUS extension", RFC 4315, December 2005. [7] Crocker, D. and P. Overell, "Augmented BNF for Syntax Specifications: ABNF", RFC 4234, October 2005. [8] Melnikov, A. and C. Daboo, "Collected Extensions to IMAP4 ABNF", RFC 4466, April 2006. Author's Address Peter W. Resnick QUALCOMM Incorporated 5775 Morehouse Drive San Diego, CA 92121-1714 US Phone: +1 858 651 4478 EMail: URI: Full Copyright Statement Copyright (C) The Internet Society (2006). This document is subject to the rights, licenses and restrictions contained in BCP 78, and except as set forth therein, the authors retain all their rights. This document and the information contained herein are provided on an "AS IS" basis and THE CONTRIBUTOR, THE ORGANIZATION HE/SHE REPRESENTS OR IS SPONSORED BY (IF ANY), THE INTERNET SOCIETY AND THE INTERNET ENGINEERING TASK FORCE DISCLAIM ALL WARRANTIES, EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE INFORMATION HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED WARRANTIES OF MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE. Intellectual Property The IETF takes no position regarding the validity or scope of any Intellectual Property Rights or other rights that might be claimed to pertain to the implementation or use of the technology described in this document or the extent to which any license under such rights might or might not be available; nor does it represent that it has made any independent effort to identify any such rights. Information on the procedures with respect to rights in RFC documents can be found in BCP 78 and BCP 79. Copies of IPR disclosures made to the IETF Secretariat and any assurances of licenses to be made available, or the result of an attempt made to obtain a general license or permission for the use of such proprietary rights by implementers or users of this specification can be obtained from the IETF on-line IPR repository at The IETF invites any interested party to bring to its attention any copyrights, patents or patent applications, or other proprietary rights that may cover technology that may be required to implement this standard. Please address the information to the IETF at Acknowledgement Funding for the RFC Editor function is provided by the IETF Administrative Support Activity (IASA).