Internet Message Access Protocol (IMAP) - MOVE Extension
One IMAP command that moves messages between mailboxes without the copy, flag and expunge dance.
Gulbrandsen, A. and N. Freed, Ed., "Internet Message Access Protocol (IMAP) - MOVE Extension", RFC 6851, DOI 10.17487/RFC6851, January 2013, <https://www.rfc-editor.org/info/rfc6851>.
@misc{rfc6851,
series = {Request for Comments},
number = 6851,
howpublished = {RFC 6851},
publisher = {RFC Editor},
doi = {10.17487/RFC6851},
url = {https://www.rfc-editor.org/info/rfc6851},
author = {A. Gulbrandsen and N. Freed},
title = {{Internet Message Access Protocol (IMAP) - MOVE Extension}},
pagetotal = 8,
year = 2013,
month = jan,
} RFC text
Contents
Internet Engineering Task Force (IETF) A. Gulbrandsen
Request for Comments: 6851
Category: Standards Track N. Freed, Ed.
ISSN: 2070-1721 Oracle
January 2013
Internet Message Access Protocol (IMAP) - MOVE Extension
Abstract
This document defines an IMAP extension consisting of two new
commands, MOVE and UID MOVE, that are used to move messages from one
mailbox to another.
Status of This Memo
This is an Internet Standards Track document.
This document is a product of the Internet Engineering Task Force
(IETF). It represents the consensus of the IETF community. It has
received public review and has been approved for publication by the
Internet Engineering Steering Group (IESG). Further information on
Internet Standards is available in Section 2 of RFC 5741.
Information about the current status of this document, any errata,
and how to provide feedback on it may be obtained at
http://www.rfc-editor.org/info/rfc6851.
Copyright Notice
Copyright (c) 2013 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents
(http://trustee.ietf.org/license-info) in effect on the date of
publication of this document. Please review these documents
carefully, as they describe your rights and restrictions with respect
to this document. Code Components extracted from this document must
include Simplified BSD License text as described in Section 4.e of
the Trust Legal Provisions and are provided without warranty as
described in the Simplified BSD License.
1. Introduction
This document defines an IMAP [RFC3501] extension to facilitate
moving messages from one mailbox to another. This is accomplished by
defining a new MOVE command and extending the UID command to allow
UID MOVE.
A move function is not provided in the base IMAP specification, so
clients have instead had to use a combination of the COPY, STORE, and
EXPUNGE commands to perform this very common operation.
Implementors have long pointed out some shortcomings with this
approach. Because the moving of a message is not an atomic process,
interruptions can leave messages in intermediate states. Because
multiple clients can be accessing the mailboxes at the same time,
clients can see messages in intermediate states even without
interruptions. If the source mailbox contains other messages that
are flagged for deletion, the third step can have the side effect of
expunging more than just the set of moved messages. Additionally,
servers with certain types of back-end message stores might have
efficient ways of moving messages, which don't involve the actual
copying of data. Such efficiencies are often not available to the
COPY/STORE/EXPUNGE process.
The MOVE extension is present in any IMAP implementation that returns
"MOVE" as one of the supported capabilities to the CAPABILITY
command.
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 specified using ABNF [RFC5234].
Example lines prefaced by "C:" are sent by the client and ones
prefaced by "S:" by the server.
3. MOVE and UID MOVE
3.1. MOVE Command
Arguments: sequence set
mailbox name
Responses: no specific responses for this command
Result: OK - move completed
NO - move error: can't move those messages or to that name
BAD - command unknown or arguments invalid
3.2. UID MOVE Command
This extends the first form of the UID command (see [RFC3501],
Section 6.4.8) to add the MOVE command defined above as a valid
argument.
3.3. Semantics of MOVE and UID MOVE
The MOVE command takes two arguments: a message set (sequence numbers
for MOVE, UIDs for UID MOVE) and a named mailbox. Each message
included in the set is moved, rather than copied, from the selected
(source) mailbox to the named (target) mailbox.
This means that a new message is created in the target mailbox with a
new UID, the original message is removed from the source mailbox, and
it appears to the client as a single action. This has the same
effect for each message as this sequence:
1. [UID] COPY
2. [UID] STORE +FLAGS.SILENT \DELETED
3. UID EXPUNGE
Although the effect of the MOVE is the same as the preceding steps,
the semantics are not identical: The intermediate states produced by
those steps do not occur, and the response codes are different. In
particular, though the COPY and EXPUNGE response codes will be
returned, response codes for a STORE MUST NOT be generated and the
\DELETED flag MUST NOT be set for any message.
Because a MOVE applies to a set of messages, it might fail partway
through the set. Regardless of whether the command is successful in
moving the entire set, each individual message SHOULD either be moved
or unaffected. The server MUST leave each message in a state where
it is in at least one of the source or target mailboxes (no message
can be lost or orphaned). The server SHOULD NOT leave any message in
both mailboxes (it would be bad for a partial failure to result in a
bunch of duplicate messages). This is true even if the server
returns a tagged NO response to the command.
Because of the similarity of MOVE to COPY, extensions that affect
COPY affect MOVE in the same way. Response codes such as TRYCREATE
(see [RFC3501], Section 6.4.7), as well as those defined by
extensions, are sent as appropriate. See Section 4 for more
information about how MOVE interacts with other IMAP extensions.
An example:
C: a UID MOVE 42:69 foo
S: * OK [COPYUID 432432 42:69 1202:1229]
S: * 22 EXPUNGE
S: (more expunges)
S: a OK Done
Note that the server may send unrelated EXPUNGE responses as well, if
any happen to have been expunged at the same time; this is normal
IMAP operation.
Implementers will need to read [RFC4315] to understand what UID
EXPUNGE does, though full implementation of [RFC4315] is not
necessary.
Note that moving a message to the currently selected mailbox (that
is, where the source and target mailboxes are the same) is allowed
when copying the message to the currently selected mailbox is
allowed.
The server may send EXPUNGE (or VANISHED) responses before the tagged
response, so the client cannot safely send more commands with message
sequence number arguments while the server is processing MOVE or UID
MOVE.
Both MOVE and UID MOVE can be pipelined with other commands, but care
has to be taken. Both commands modify sequence numbers and also
allow unrelated EXPUNGE responses. The renumbering of other messages
in the source mailbox following any EXPUNGE response can be
surprising and makes it unsafe to pipeline any command that relies on
message sequence numbers after a MOVE or UID MOVE. Similarly, MOVE
cannot be pipelined with a command that might cause message
renumbering. See [RFC3501], Section 5.5, for more information about
ambiguities as well as handling requirements for both clients and
servers.
4. Interaction with Other Extensions
This section describes how MOVE interacts with some other IMAP
extensions.
4.1. RFC 2087, QUOTA
The QUOTA extension (defined by [RFC2087]) may interact with MOVE on
some servers, in the sense that a MOVE command may succeed where COPY
would cause a quota overrun.
4.2. RFC 4314, Access Control List (ACL)
The ACL rights [RFC4314] required for MOVE and UID MOVE are the union
of the ACL rights required for UID STORE, UID COPY, and UID EXPUNGE.
4.3. RFC 4315, UIDPLUS
Servers supporting UIDPLUS [RFC4315] SHOULD send COPYUID in response
to a UID MOVE command. For additional information see Section 3 of
[RFC4315].
Servers implementing UIDPLUS are also advised to send the COPYUID
response code in an untagged OK before sending EXPUNGE or moved
responses. (Sending COPYUID in the tagged OK, as described in the
UIDPLUS specification, means that clients first receive an EXPUNGE
for a message and afterwards COPYUID for the same message. It can be
unnecessarily difficult to process that sequence usefully.)
4.4. RFC 5162, QRESYNC
The QRESYNC extension [RFC5162] states that the server SHOULD send
VANISHED rather than EXPUNGE in response to the UID EXPUNGE command.
The same requirement applies to MOVE, and a QRESYNC-enabled client
needs to handle both VANISHED and EXPUNGE responses to a UID MOVE
command.
If the server is capable of storing modification sequences for the
selected mailbox, it MUST increment the per-mailbox mod-sequence if
at least one message was permanently moved due to the execution of
the MOVE/UID MOVE command. For each permanently removed message, the
server MUST remember the incremented mod-sequence and corresponding
UID. If at least one message was moved, the server MUST send the
updated per-mailbox modification sequence using the HIGHESTMODSEQ
response code (defined in [RFC4551]) in the tagged or untagged OK
response.
When one or more messages are moved to a target mailbox, if the
server is capable of storing modification sequences for the mailbox,
the server MUST generate and assign new modification sequence numbers
to the moved messages that are higher than the highest modification
sequence of the messages originally in the mailbox.
4.5. IMAP Events in Sieve
MOVE applies to IMAP events in Sieve [RFC6785] in the same way as
COPY does. Therefore, MOVE can cause a Sieve script to be invoked
with the imap.cause set to "COPY". Because MOVE does not cause flags
to be changed, a MOVE command will not result in a script invocation
with the imap.cause set to "FLAG".
5. Formal Syntax
The following syntax specification uses the Augmented Backus-Naur
Form (ABNF) notation as specified in [RFC5234]. [RFC3501] defines
the non-terminals "capability", "command-select", "sequence-set", and
"mailbox".
Except as noted otherwise, all alphabetic characters are case
insensitive. The use of upper or lower case characters to define
token strings is for editorial clarity only. Implementations MUST
accept these strings in a case-insensitive fashion.
capability =/ "MOVE"
command-select =/ move
move = "MOVE" SP sequence-set SP mailbox
uid = "UID" SP (copy / fetch / search / store / move)
6. Security Considerations
MOVE does not introduce any new capabilities to IMAP, and this limits
the security impact. However, the transactional semantics of MOVE
may interact with specific implementations in ways that could have
unexpected consequences. For example, moving messages between
mailboxes under the quota root may require temporary suspension of
quota checking.
An additional area of concern is interaction with antispam,
antivirus, and other security scanning and auditing mechanisms.
Different mailboxes may have different security policies that could
interact with MOVE in complex ways. Scanning with updated rules may
also be required when messages are moved even when the underlying
policy has not changed.
MOVE does relieve a problem with the base specification, since client
authors currently have to devise and implement complicated algorithms
to handle partial failures of the STORE/COPY/EXPUNGE trio.
Incomplete or improper implementation of these algorithms can lead to
mail loss.
7. IANA Considerations
The IANA has added MOVE to the "IMAP 4 Capabilities" registry,
<http://www.iana.org/assignments/imap4-capabilities>.
8. Acknowledgments
This document is dedicated to the memory of Mark Crispin, the
inventor of the IMAP protocol, author of the IMAP protocol
specification [RFC3501], and contributor to many other email
specifications in the IETF.
An extension like this has been proposed many times, by many people.
This document is based on several of those proposals, most recently
that by Witold Krecicki. Witold, Benoit Claise, Adrien W. de Croy,
Stephen Farrell, Bron Gondwana, Dan Karp, Christian Ketterer, Murray
Kucherawy, Jan Kundrat, Barry Leiba, Alexey Melnikov, Kathleen
Moriarty, Zoltan Ordogh, Pete Resnick, Timo Sirainen, Michael
Slusarz, and others provided valuable comments.
9. References
9.1. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC3501] Crispin, M., "INTERNET MESSAGE ACCESS PROTOCOL - VERSION
4rev1", RFC 3501, March 2003.
[RFC4314] Melnikov, A., "IMAP4 Access Control List (ACL) Extension",
RFC 4314, December 2005.
[RFC4315] Crispin, M., "Internet Message Access Protocol (IMAP) -
UIDPLUS extension", RFC 4315, December 2005.
[RFC4551] Melnikov, A. and S. Hole, "IMAP Extension for Conditional
STORE Operation or Quick Flag Changes Resynchronization",
RFC 4551, June 2006.
[RFC5162] Melnikov, A., Cridland, D., and C. Wilson, "IMAP4
Extensions for Quick Mailbox Resynchronization", RFC 5162,
March 2008.
[RFC5234] Crocker, D. and P. Overell, "Augmented BNF for Syntax
Specifications: ABNF", STD 68, RFC 5234, January 2008.
9.2. Informative References
[RFC2087] Myers, J., "IMAP4 QUOTA extension", RFC 2087,
January 1997.
[RFC6785] Leiba, B., "Support for Internet Message Access Protocol
(IMAP) Events in Sieve", RFC 6785, November 2012.
Authors' Addresses
Arnt Gulbrandsen
Schweppermannstr. 8
D-81671 Muenchen
Germany
Fax: +49 89 4502 9758
EMail: arnt@gulbrandsen.priv.no
Ned Freed (editor)
Oracle
800 Royal Oaks
Monrovia, CA 91016-6347
USA
EMail: ned+ietf@mrochek.com
AI ELI5
On this page
§1Why IMAP Needs a Move Command
RFC 6851 defines an extension to IMAP (RFC 3501) that makes it easy to move messages from one mailbox to another. It adds a new MOVE command and extends the UID command so that UID MOVE works too.
The base IMAP specification has no move function. Clients have had to combine COPY, STORE and EXPUNGE to do this very common operation. A server advertises the extension by returning MOVE as a capability in response to the CAPABILITY command.
Normally, to move a toy to another box you would make a copy and then throw the first one away. This gives you a way to just pick it up and put it in the other box.
§2Problems with Copy, Store and Expunge
The RFC lists the shortcomings of the old three-step approach. Moving a message that way is not an atomic process, so an interruption can leave messages in intermediate states. Because several clients can open the same mailboxes at the same time, other clients can see those intermediate states even without interruptions.
There is a side effect too: if the source mailbox holds other messages already flagged for deletion, the final expunge step can remove more than just the moved messages. Finally, servers with certain back-end message stores may have efficient ways to move messages without copying any data, and the copy-and-delete sequence cannot use them.
Copy-then-delete is like photocopying a page and then tearing out the original. If someone walks in halfway, they see two pages, and you might tear out the wrong ones.
§3How MOVE and UID MOVE Work
MOVE takes two arguments: a message set and a mailbox name. For MOVE the set holds sequence numbers, and for UID MOVE it holds UIDs. Each message in the set is moved from the selected (source) mailbox to the named (target) mailbox. A new message with a new UID is created in the target, the original is removed from the source, and the client sees it as a single action. The result is OK if the move completed, NO if the messages cannot be moved or cannot go to that name, and BAD if the command is unknown or its arguments invalid.
The effect per message matches [UID] COPY, then [UID] STORE +FLAGS.SILENT \DELETED, then UID EXPUNGE, but the semantics are not identical. The intermediate states do not occur, and the server MUST NOT generate STORE response codes or set the \DELETED flag on any message. COPY and EXPUNGE response codes are still returned.
Because a move covers a set, it may fail partway. Each message SHOULD either be moved or left alone. The server MUST leave every message in at least one of the two mailboxes, so nothing is lost, and SHOULD NOT leave any message in both. This holds even when the server returns a tagged NO.
Extensions that affect COPY affect MOVE in the same way, so response codes such as TRYCREATE are sent as appropriate. Moving to the currently selected mailbox is allowed when copying there is allowed.
You tell the server which messages and which folder. It puts each one in the new folder and removes it from the old one, and it promises no message ever ends up lost.
§4Example and Pipelining Cautions
The RFC gives this exchange, in which the server sends a COPYUID code, then a series of EXPUNGE responses, then the tagged OK. The server may also send unrelated EXPUNGE responses if other messages happen to be expunged at the same time, which is normal IMAP operation.
The server may send EXPUNGE (or VANISHED) responses before the tagged response. A client therefore cannot safely send more commands with message sequence number arguments while the server is processing a move. Both commands can be pipelined with other commands, but care is needed, because the renumbering that follows an EXPUNGE makes it unsafe to pipeline any command that relies on sequence numbers after a move. Likewise, a move cannot be pipelined with a command that might cause renumbering.
Implementers need to read RFC 4315 to understand UID EXPUNGE, though full implementation of it is not necessary.
While the server is tidying up, the numbering of the messages in the folder can shift. So the client should not point at messages by position until the server says it is done.
- a UID MOVE 42:69 fooMove UIDs 42 to 69 to mailbox foo.
- * OK [COPYUID 432432 42:69 1202:1229]New UIDs in the target mailbox.
- * 22 EXPUNGE (and more)Originals removed from the source.
- a OK Done
§5Interaction with Other Extensions
Section 4 of the RFC covers how MOVE works alongside other IMAP extensions.
- QUOTA (RFC 2087): a MOVE may succeed where a COPY would cause a quota overrun.
- ACL (RFC 4314): the rights required are the union of those for
UID STORE,UID COPYandUID EXPUNGE. - UIDPLUS (RFC 4315): servers SHOULD send
COPYUIDin response toUID MOVE, and are advised to send it in an untagged OK before the EXPUNGE or moved responses, so clients do not see an expunge before the matching COPYUID. - QRESYNC (RFC 5162): the server SHOULD send
VANISHEDrather than EXPUNGE, as for UID EXPUNGE, and the client must handle both. A server that stores modification sequences MUST increment the per-mailbox mod-sequence when messages are moved, sendHIGHESTMODSEQ, and give moved messages new mod-sequences higher than the highest in the target mailbox. - Sieve events (RFC 6785): MOVE is treated like COPY, so a script can be invoked with
imap.causeset toCOPY, but neverFLAG, because MOVE does not change flags.
MOVE is meant to play nicely with the other add-on features of IMAP, such as storage limits, permissions and keeping a client in sync.
§6Formal Syntax
The RFC defines the syntax in ABNF. It adds MOVE to the list of capabilities and to the commands usable once a mailbox is selected, and it adds move as an allowed argument of UID. The command is MOVE, a space, a sequence set, a space and a mailbox name. Token strings are case insensitive and implementations MUST accept them that way.
The grammar just says what a MOVE line looks like: the word MOVE, which messages, and where to.
§7Security and IANA Notes
MOVE adds no new capabilities to IMAP, which limits its security impact. Its transactional semantics may still interact with implementations in unexpected ways. For example, moving messages between mailboxes under the quota root may require temporary suspension of quota checking. Different mailboxes may also have different antispam, antivirus or auditing policies, and messages may need rescanning with updated rules after a move.
On the positive side, MOVE removes the need for complicated client algorithms to handle partial failures of the STORE, COPY and EXPUNGE trio, and incomplete versions of those algorithms can lead to mail loss. IANA added MOVE to the IMAP 4 Capabilities registry.
Moving mail is safer than the old way, but the server has to make sure its spam and virus checks still apply in the new folder.