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IETF RFC 5788
IMAP4 Keyword Registry
Last modified on Thursday, March 4th, 2010
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Internet Engineering Task Force (IETF) A. Melnikov
Request for Comments: 5788 D. Cridland
Category: Standards Track Isode Limited
ISSN: 2070-1721 March 2010
IMAP4 Keyword Registry
 Abstract
The aim of this document is to establish a new IANA registry for IMAP
keywords and to define a procedure for keyword registration, in order
to improve interoperability between different IMAP clients.
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/RFC 5788.
Copyright Notice
Copyright (c) 2010 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.
Melnikov & Cridland Standards Track PAGE 1 
RFC 5788 IMAP4 Keyword Registry March 2010
Table of Contents
1. Introduction ....................................................2
2. Conventions Used in This Document ...............................2
3. IANA Considerations .............................................3
3.1. Review Guidelines for the Designated Expert Reviewer .......4
3.2. Comments on IMAP Keywords' Registrations ...................5
3.3. Change Control .............................................5
3.4. Initial Registrations ......................................6
3.4.1. $MDNSent IMAP Keyword Registration ..................6
3.4.2. $Forwarded IMAP Keyword Registration ................7
3.4.3. $SubmitPending IMAP Keyword Registration ............8
3.4.4. $Submitted IMAP Keyword Registration ................9
4. Security Considerations ........................................10
5. Acknowledgements ...............................................10
6. References .....................................................10
6.1. Normative References ......................................10
6.2. Informative References ....................................11
1. Introduction
IMAP keywords [RFC 3501] are boolean named flags that can be used by
clients to annotate messages in an IMAP mailbox. Although IMAP
keywords are an optional feature of IMAP, the majority of IMAP
servers can store arbitrary keywords. Many mainstream IMAP clients
use a limited set of specific keywords, and some can manage (create,
edit, display) arbitrary IMAP keywords.
Over the years, some IMAP keywords have become de-facto standards,
with some specific semantics associated with them. In some cases,
different client implementors decided to define and use keywords with
different names, but the same semantics. Some server implementors
decided to map such keywords automatically in order to improve cross-
client interoperability.
In other cases, the same keywords have been used with different
semantics, thus causing interoperability problems.
This document attempts to prevent further incompatible uses of IMAP
keywords by establishing an "IMAP Keywords" registry and allocating a
special prefix for standardized keywords.
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 RFC 2119 [Kwds].
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RFC 5788 IMAP4 Keyword Registry March 2010
3. IANA Considerations
IANA has established a new registry for IMAP keywords.
Registration of an IMAP keyword is requested by filling in the
following template and following the instructions on the IANA pages
for the registry to submit it to IANA:
Subject: Registration of IMAP keyword X
IMAP keyword name:
Purpose (description):
Private or Shared on a server: (One of PRIVATE, SHARED, or BOTH.
PRIVATE means that each different
user has a distinct copy of the
keyword. SHARED means that all
different users see the same value of
the keyword. BOTH means that an IMAP
server can have the keyword as either
private or shared.)
Is it an advisory keyword or may it cause an automatic action:
When/by whom the keyword is set/cleared:
Related keywords: (for example, "mutually exclusive with keywords Y
and Z")
Related IMAP capabilities:
Security considerations:
Published specification (recommended):
Person & email address to contact for further information:
Intended usage: (One of COMMON, LIMITED USE, or DEPRECATED (i.e.,
not recommended for use))
Owner/Change controller: (MUST be "IESG" for any "common use"
keyword registration specified in an IETF
Review document. See definition of "common
use" below in this section. When the
Owner/Change controller is not a
Standardization Organization, the
registration request MUST make it clear if
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RFC 5788 IMAP4 Keyword Registry March 2010
the registration is controlled by a
company, or the individual performing the
registration.)
Note: (Any other information that the author deems interesting
may be added here, for example, if the keyword(s) is
supported by existing clients.)
Registration of an IMAP keyword requires Expert Review [RFC 5226].
Registration of any IMAP keyword is initiated by posting a
registration request to the Message Organization WG mailing list
<morg@ietf.org> (or its replacement as chosen by the responsible
Application Area Director) and CCing IANA (<iana@iana.org>). After
allowing for at least two weeks for community input on the designated
mailing list, the expert will determine the appropriateness of the
registration request and either approve or disapprove the request
with notice to the requestor, the mailing list, and IANA. Any
refusal must come with a clear explanation.
The IESG appoints one or more Expert Reviewers for the IMAP keyword
registry established by this document.
The Expert Reviewer should strive for timely reviews. The Expert
Reviewer should take no longer than six weeks to make and announce
the decision, or notify the mailing list that more time is required.
Decisions (or lack of) made by an Expert Reviewer can be first
appealed to Application Area Directors and, if the appellant is not
satisfied with the response, to the full IESG.
There are two types of IMAP keywords in the "IMAP Keywords" registry:
intended for "common use" and vendor-/organization-specific use (also
known as "limited use"). An IMAP keyword is said to be for "common
use" if it is reasonably expected to be implemented in at least two
independent client implementations. The two types of IMAP keywords
have different levels of requirements for registration (see below).
3.1. Review Guidelines for the Designated Expert Reviewer
Expert Reviewers should focus on the following requirements.
Registration of a vendor-/organization-specific ("limited use") IMAP
keyword is easier. The Expert Reviewer only needs to check that the
requested name doesn't conflict with an already registered name, and
that the name is not too generic, misleading, etc. The Expert
Reviewer MAY request the IMAP keyword name change before approving
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RFC 5788 IMAP4 Keyword Registry March 2010
the registration. The Expert Reviewer SHOULD refuse a registration
if there is an already registered IMAP keyword that serves the same
purpose, but has a different name.
When registering an IMAP keyword for "common use", the Expert
Reviewer performs the checks described for vendor-/
organization-specific IMAP keywords, plus additional checks as
detailed below.
Keywords intended for "common use" SHOULD start with the "$" prefix.
(Note that this is a SHOULD because some of the commonly used IMAP
keywords in widespread use don't follow this convention.)
IMAP keywords intended for "common use" SHOULD be standardized in
IETF Review [RFC 5226] documents. (Note that IETF Review documents
still require Expert Review.)
Values in the "IMAP Keywords" IANA registry intended for "common use"
must be clearly documented and likely to ensure interoperability.
They must be useful, not harmful to the Internet. In cases when an
IMAP keyword being registered is already deployed, Expert Reviewers
should favor registering it over requiring perfect documentation
and/or requesting a change to the name of the IMAP keyword.
The Expert Reviewer MAY automatically "upgrade" registration requests
for a "limited use" IMAP keyword to "common use" level. The Expert
Reviewer MAY also request that a registration targeted for "common
use" be registered as "limited use" instead.
3.2. Comments on IMAP Keywords' Registrations
Comments on registered IMAP keywords should be sent to both the
"owner" of the mechanism and to the mailing list designated to IMAP
keyword review (see Section 3). This improves the chances of getting
a timely response.
Submitters of comments may, after a reasonable attempt to contact the
owner and after soliciting comments on the IMAP mailing list, request
the designated Expert Reviewer to attach their comment to the IMAP
keyword registration itself. The procedure is similar to requesting
an Expert Review for the affected keyword.
3.3. Change Control
Once an IMAP keyword registration has been published by IANA, the
owner may request a change to its definition. The change request
(including a change to the "intended usage" field) follows the same
procedure as the initial registration request, with the exception of
Melnikov & Cridland Standards Track PAGE 5
RFC 5788 IMAP4 Keyword Registry March 2010
changes to the "Person & email address to contact for further
information" and "Owner/Change controller" fields. The latter can be
changed by the owner by informing IANA; this can be done without
discussion or review.
The IESG may reassign responsibility for an IMAP keyword. The most
common case of this will be to enable clarifications to be made to
keywords where the owner of the registration has died, moved out of
contact, or is otherwise unable to make changes that are important to
the community.
IMAP keyword registrations MUST NOT be deleted; keywords that are no
longer believed appropriate for use can be declared DEPRECATED by a
change to their "intended usage" field.
The IESG is considered the owner of all "common use" IMAP keywords
that are published in an IETF Review document.
3.4. Initial Registrations
IANA has registered the IMAP keywords specified in following
subsections in the registry established by this document.
3.4.1. $MDNSent IMAP Keyword Registration
Subject: Registration of IMAP keyword $MDNSent
IMAP keyword name: $MDNSent
Purpose (description): Specifies that a Message Disposition
Notification (MDN) must not be sent for any
message annotated with the $MDNSent IMAP
keyword.
Private or Shared on a server: SHARED
Is it an advisory keyword or may it cause an automatic action:
This keyword can cause automatic action by
the client. See [RFC 3503] for more details.
When/by whom the keyword is set/cleared:
This keyword is set by an IMAP client when it
decides to act on an MDN request, or when
uploading a sent or draft message. It can
also be set by a delivery agent. Once set,
the flag SHOULD NOT be cleared.
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RFC 5788 IMAP4 Keyword Registry March 2010
Related keywords: None
Related IMAP capabilities: None
Security considerations: See Section 6 of [RFC 3503]
Published specification (recommended): [RFC 3503]
Person & email address to contact for further information:
Alexey Melnikov <alexey.melnikov@isode.com>
Intended usage: COMMON
Owner/Change controller: IESG
Note:
3.4.2. $Forwarded IMAP Keyword Registration
Subject: Registration of the IMAP keyword $Forwarded
IMAP keyword name: $Forwarded
Purpose (description): $Forwarded is used by several IMAP clients to
specify that the message was resent to
another email address, embedded within or
attached to a new message. This keyword is
set by the mail client when it successfully
forwards the message to another email
address. Typical usage of this keyword is to
show a different (or additional) icon for a
message that has been forwarded.
Private or Shared on a server: BOTH
Is it an advisory keyword or may it cause an automatic action:
advisory
When/by whom the keyword is set/cleared:
This keyword can be set by either a delivery
agent or a mail client. Once set, the flag
SHOULD NOT be cleared. Notes: There is no
way to tell if a message with $Forwarded
keyword set was forwarded more than once.
Related keywords: None
Related IMAP capabilities: None
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RFC 5788 IMAP4 Keyword Registry March 2010
Security considerations: A server implementing this keyword as a
shared keyword may disclose that a
confidential message was forwarded.
Published specification (recommended): [RFC 5550]
Person & email address to contact for further information:
Alexey Melnikov <alexey.melnikov@isode.com>
Intended usage: COMMON
Owner/Change controller: IESG
Note:
3.4.3. $SubmitPending IMAP Keyword Registration
Subject: Registration of IMAP keyword $SubmitPending
IMAP keyword name: $SubmitPending
Purpose (description): The $SubmitPending IMAP keyword designates
the message as awaiting to be submitted.
This keyword allows storing messages waiting
to be submitted in the same mailbox where
messages that were already submitted and/or
are being edited are stored.
A message that has both $Submitted and
$SubmitPending IMAP keywords set is a message
being actively submitted.
Private or Shared on a server: SHARED
Is it an advisory keyword or may it cause an automatic action:
This keyword can cause automatic action by
the client. See Section 5.10 of [RFC 5550]
for more details.
When/by whom the keyword is set/cleared:
This keyword is set by a mail client when it
decides that the message needs to be sent
out.
Related keywords: $Submitted
Related IMAP capabilities: None
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RFC 5788 IMAP4 Keyword Registry March 2010
Security considerations: A server implementing this keyword as a
shared keyword may disclose that a
confidential message is scheduled to be
sent out or is being actively sent out.
Published specification (recommended): [RFC 5550]
Person & email address to contact for further information:
Alexey Melnikov <alexey.melnikov@isode.com>
Intended usage: COMMON
Owner/Change controller: IESG
Note:
3.4.4. $Submitted IMAP Keyword Registration
Subject: Registration of IMAP keyword $Submitted
IMAP keyword name: $Submitted
Purpose (description): The $Submitted IMAP keyword designates the
message as being sent out.
A message that has both $Submitted and
$SubmitPending IMAP keywords set is a message
being actively submitted.
Private or Shared on a server: SHARED
Is it an advisory keyword or may it cause an automatic action:
This keyword can cause automatic action by
the client. See Section 5.10 of [RFC 5550]
for more details.
When/by whom the keyword is set/cleared:
This keyword is set by a mail client when it
decides to start sending it.
Related keywords: $SubmitPending
Related IMAP capabilities: None
Security considerations: A server implementing this keyword as a
shared keyword may disclose that a
confidential message was sent or is being
actively sent out.
Melnikov & Cridland Standards Track PAGE 9
RFC 5788 IMAP4 Keyword Registry March 2010
Published specification (recommended): [RFC 5550]
Person & email address to contact for further information:
Alexey Melnikov <alexey.melnikov@isode.com>
Intended usage: COMMON
Owner/Change controller: IESG
Note:
4. Security Considerations
IMAP keywords are one of the base IMAP features [RFC 3501]. This
document doesn't change their behavior, so it does not add new
security issues.
A particular IMAP keyword might have specific security
considerations, which are documented in the IMAP keyword
registration template standardized by this document.
5. Acknowledgements
The creation of this document was prompted by one of many discussions
on the IMAP mailing list.
John Neystadt co-authored the first version of this document.
Special thanks to Chris Newman, David Harris, Lyndon Nerenberg, Mark
Crispin, Samuel Weiler, Alfred Hoenes, Lars Eggert, and Cullen
Jennings for reviewing different versions of this document. However,
all errors or omissions must be attributed to the authors of this
document.
The authors would also like to thank the developers of Mozilla mail
clients for providing food for thought.
6. References
6.1. Normative References
[Kwds] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC 3501] Crispin, M., "INTERNET MESSAGE ACCESS PROTOCOL - VERSION
4rev1", RFC 3501, March 2003.
Melnikov & Cridland Standards Track PAGE 10
RFC 5788 IMAP4 Keyword Registry March 2010
[RFC 5226] Narten, T. and H. Alvestrand, "Guidelines for Writing an
IANA Considerations Section in RFCs", BCP 26, RFC 5226,
May 2008.
6.2. Informative References
[RFC 3503] Melnikov, A., "Message Disposition Notification (MDN)
profile for Internet Message Access Protocol (IMAP)",
RFC 3503, March 2003.
[RFC 5550] Cridland, D., Melnikov, A., and S. Maes, "The Internet
Email to Support Diverse Service Environments (Lemonade)
Profile", RFC 5550, August 2009.
Authors' Addresses
Alexey Melnikov
Isode Limited
5 Castle Business Village
36 Station Road
Hampton, Middlesex TW12 2BX
UK
EMail: Alexey.Melnikov@isode.com
URI: http://www.melnikov.ca/
Dave Cridland
Isode Limited
5 Castle Business Village
36 Station Road
Hampton, Middlesex TW12 2BX
UK
EMail: dave.cridland@isode.com
Melnikov & Cridland Standards Track PAGE 11
IMAP4 Keyword Registry
RFC TOTAL SIZE: 22497 bytes
PUBLICATION DATE: Thursday, March 4th, 2010
LEGAL RIGHTS: The IETF Trust (see BCP 78)
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