Internet Draft: Common IMAP keywords A. Melnikov
Document: draft-melnikov-imap-keywords-03.txt Isode Ltd.
Expires: March 2004 September 2003
Intended category: Informational
Registration of common IMAP keywords
Status of this Memo
This document is an Internet-Draft and is in full conformance with
all provisions of Section 10 of RFC2026. Internet-Drafts are working
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A revised version of this draft document will be submitted to the RFC
editor as an Informational document. Discussion and suggestions for
improvement are requested, and should be sent to the IMAP4 Mailing
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Copyright Notice
Copyright (C) The Internet Society (2002-2003). All Rights Reserved.
Table of Contents
0. To do ....................................................... 1
1. Abstract .................................................... 2
2. Conventions Used in this Document ........................... 2
3. IMAP keyword registrations .................................. 3
3.1 $Forwarded .................................................. 3
3.2 Keywords related to junk filtering .......................... X
3.3 $Work ....................................................... X
3.4 $Personal ................................................... X
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3.5 $ShouldReply ................................................ X
3.6 $Important .................................................. X
4. Security Considerations ..................................... 4
5. Known issues with mail clients .............................. 4
6. Formal Syntax ............................................... 4
7. Acknowledgments ............................................. 5
8. References .................................................. 5
9. Author's Addresses .......................................... 5
10. Full Copyright Statement .................................... 6
1. Abstract
The aim of this document is to document some common [IMAP4] keywords
for the purpose of improving interoperability between different IMAP
mail clients. The document both documents some keywords already in
use, as well as introduces several new ones.
2. Conventions Used in this Document
"C:" and "S:" in examples show lines sent by the client and server
respectively.
The keywords "MUST", "MUST NOT", "SHOULD", "SHOULD NOT", and "MAY" in
this document when typed in uppercase are to be interpreted as
defined in "Key words for use in RFCs to Indicate Requirement Levels"
[KEYWORDS].
<<Irreversible action - any action that causes alteration of the
message, its deletion or move to a different mailbox, generation of
an automatic reply based on the message (bouncing to sender,
notifying administrator, etc.). Actions like hiding the message,
grouping messages or displaying different icon(s) are not considered
to be "irreversible actions".>>
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3. IMAP keyword registrations
3.1 $Forwarded
Purpose: $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: either
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
either by a delivery agent or a mail client. Once set the flag SHOULD
NOT be cleared.
Notes: There is no way to tell if the message with $Forwarded flags
was forwarded more than once.
Corresponding ANNOTATE attribute: "/message/flags/$forwarded"
(default mapping as described in ANNOTATE)
Additional IMAP capabilities: none.
Security Considerations: A server implementing this keyword as a
shared keyword, may disclose that a confidential message was
forwarded.
3.2 Keywords related to junk filtering
This section defines two groups of keywords related to mail
filtering.
3.2.1 $AutoJunk/$AutoNotJunk/$AutoMaybeJunk
Purpose: An automated system (delivery agent, mail filtering software
or IMAP4/POP3 server) may mark a message with one of the following
keywords:
$AutoJunk - a message is most likely to contain junk;
$AutoNotJunk - a message is most likely not to contain junk;
$AutoMaybeJunk - a message may contain junk;
$AutoMaybeJunk, for example, may be used by a Bayesian filtering
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software that can't classify a message with certainty as being either
junk or not junk.
These keyword can be used to mark, group or hide ($AutoJunk)
offensive messages. However no automatic irreversible action (in
particular message deletion, bouncing, moving into a different
mailbox) should be allowed on messages marked as $AutoJunk.
Private or Shared on a server: may be shared. SHOULD be shared when
automatically set by a delivery agent or an IMAP4/POP3 server.
Is it an advisory keyword or may it cause an automatic action:
advisory
When/by whom the keyword is set/cleared: Any one of the described
keywords can be set automaticall either by a delivery agent, a mail
filtering software or a mail (IMAP4/POP3) server. Once set, the value
SHOULD NOT be changed. If $AutoJunk and/or $AutoMaybeJunk are used
to hide all messages with those flags from the current view, the mail
client MUST implement a mode of operation when it is possible to see
the hidden messages.
Notes: $AutoJunk, $AutoNotJunk and $AutoMaybeJunk are mutually
exclusive. If more than one of them is set for a message, the mail
client MUST treat this as if none of them is set and SHOULD remove
all of them from the IMAP server.
Notes: See also section 3.2.2 that talks how $AutoJunk, $AutoNotJunk
and $AutoMaybeJunk may interact with $Junk and $NotJunk.
Corresponding ANNOTATE attribute: "/message/flags/$autojunk"
"/message/flags/$autonotjunk"
"/message/flags/$automaybejunk"
(default mapping as described in ANNOTATE) <<Is ANNOTATE case
sensitive?>>
Additional IMAP capabilities: ?.
Security Considerations: A message marked with $AutoJunk keyword by
an automated system might not be considered junk by any or even all
users of the system. Thus, any automated actions are prohibited
based on the keywords described in this section.
3.2.2 $Junk/$NotJunk
Purpose: The user (or a delivery agent on behalf of the user) may
choose to mark a message as either definitely containing junk ($Junk)
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or definitely not containing junk ($NotJunk). This keyword can be
used to mark (and potentially move/delete messages later), group or
hide offensive messages.
If none of $Junk or $NotJunk is present for a messages, a mail client
MAY treat $AutoJunk (section 3.2.1) as $Junk and $AutoNotJunk
(section 3.2.1) as $NotJunk. However, in this case no irreversible
action (whether automatic or manual <<not sure about manual, see
example below>>) MUST be done on the message. <<For example, if the
user chooses to move all messages marked as junk to a different
mailbox, the message that only have $AutoJunk set, MUST NOT be
moved?>>
Private or Shared on a server: private
Is it an advisory keyword or may it cause an automatic action:
advisory
When/by whom the keyword is set/cleared: $Junk/$NotJunk can be set
either by a delivery agent or a mail client on users behalf. The user
must be able to change $Junk to $NotJunk or vice versa at any time.
If the mail client hides all messages with $Junk keyword from the
current view, the mail client MUST implement a mode when it is
possible to see all messages marked as $Junk.
Notes:
1). $Junk and $NotJunk are mutually exclusive. If more than one of
them is set for a message, the mail client MUST treat this as if
none of them is set and SHOULD remove all of them from the IMAP
server.
2). $Junk is also mutually exclusive with $Work and $Personal.
If more than one of them is set for a message, the mail client
MUST treat this as if none of them is set and SHOULD remove all
of them from the IMAP server.
3). The combination of $NotJunk and $AutoJunk keywords indicates that
the automated system made a false-positive.
The combination of $Junk and $AutoNotJunk keywords indicates that
the automated system made a false-negative.
Both combinations can be used to train the automated system that
set $AutoJunk/$AutoNotJunk to do a better job in classifying junk
versa not junk.
4). There are existing clients that use mutually exclusive keywords
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Junk and NotJunk to mark a message as definitely containing
/definitely non containing junk information. Use of
"Junk"/"NotJunk" is discouraged, mail clients should be using
"$Junk"/"$NotJunk" instead.
Corresponding ANNOTATE attribute: "/message/flags/$junk"
"/message/flags/$notjunk"
(default mapping as described in ANNOTATE)
Additional IMAP capabilities: ?.
Security Considerations: A message marked with $Junk keyword by one
user might not be considered junk by another (or even by the same
user under different circumstances), so no automatic action can be
initiated when this flag is set.
3.3 $Work
Purpose: The user (or a delivery agent on behalf of the user) may
choose to mark a message as related to work. This keyword can be used
to automatically move a message to a different mailbox or to group
messages.
Private or Shared on a server: private
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
either by a delivery agent or a mail client on users behalf. The user
must be able to set and unset this keyword.
Notes: This keyword is mutually exclusive with $Junk and $Personal.
If more than one of the $Junk, $Work or $Personal keywords is set for
a message, the mail client MUST treat this as if none of them is set
and SHOULD remove all of them from the IMAP server.
Note: $Work and $Personal can be used to help implement a "poor man"
multiple personalities (actually just two personalities) feature in a
mail client.
Corresponding ANNOTATE attribute: "/message/flags/$work"
(default mapping as described in ANNOTATE)
Additional IMAP capabilities: ?.
Security Considerations: <<same as $Junk>>.
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3.4 $Personal
Purpose: The user (or a delivery agent on behalf of the user) may
choose to mark a message as personal. This keyword can be used to
automatically move a message to a different mailbox or to group
messages.
Private or Shared on a server: private
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
either by a delivery agent or a mail client on users behalf. The user
must be able to set and unset this keyword.
Notes: This keyword is mutually exclusive with $Junk and $Work. If
more than one of the $Junk, $Work or $Personal keywords is set for a
message, the mail client MUST treat this as if none of them is set
and SHOULD remove all of them from the IMAP server.
Note: $Work and $Personal can be used to help implement a "poor man"
multiple personalities (actually just two personalities) feature in a
mail client.
Corresponding ANNOTATE attribute: "/message/flags/$personal"
(default mapping as described in ANNOTATE)
Additional IMAP capabilities: ?.
Security Considerations: <<same as $Junk>>.
3.5 $ShouldReply
Purpose: The user may mark a message with this keyword to specify
that the message requires a reply. One possible use of this keyword
is to group in some way all messages having this keyword. Another
possible use would be to periodically pop up a dialog reminding the
user to send a reply. When a reply to this message is sent, the mail
client MUST remove this keyword and MUST set the \Answered flag.
<<Does $Forwarded cancel $ShouldReply?>>
Private or Shared on a server: private
Is it an advisory keyword or may it cause an automatic action:
advisory(?)
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When/by whom the keyword is set/cleared: This keyword can be set
either by a delivery agent or a mail client on users behalf (for
example, "should reply to all messages from boss with particular
subject"). The user must be able to set and unset this keyword.
Notes: <<popping up a dialog asking for reply each time the client
sees this flag might be a bad UI idea>>.
Corresponding ANNOTATE attribute: "/message/flags/$shouldreply"
(default mapping as described in ANNOTATE)
Additional IMAP capabilities: none.
Security Considerations: ...
3.6 $Important
Purpose: <<[IMAP4] doesn't specify exact semantics of the \Flagged
flag. It suggests that it is up to a MUA to assign any special
meaning to it. Most intuitive meaning of \Flagged is probably to mark
that message as important. However since the same user can use
multiple MUAs from different vendors, there are some MUAs that use it
for different purposes. In order to introduce a consistent
"Important" mark a new flag is required. This document defines a new
keyword $Important for this purpose.>>
The server that recognizes $Important SHOULD automatically set it on
a message injection if the root bodypart of that message contains the
header field "Importance" with the value "High". The server SHOULD
also set this keyword if the root body part contains either a header
field "Priority" with the value of "urgent" [HEADERS] or a header
field "X-Priority" with the value "1" or "2".
Note that a header field value comparison MUST be done after removing
RFC 822 comments (see section 3.2.3 of [RFC 2822]). For example, the
"X-Priority" header field with the value "1 (Highest)" and "1" MUST
be treated the same way.
Private or Shared on a server: private
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
either by a delivery agent or a mail client on users behalf as
described above. The user must be able to set and unset this
keyword.
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<<The absence of this keyword means use the algorithm above?>>
Notes: ...
<<Describe how client should display that: use described header
fields if the $Important keyword is not set. The keyword overrides
header fields.>>
<<Add mutual exclusive $Important/$Normal/$Unimportant, so that we
can override the meaning prescribed by headers?>>
<<Separate Importance from Priority? I don't see much difference,
except for Priority controlling delivery speed, that most MTAs
probably ignore anyway. Define separate flags?>>
Corresponding ANNOTATE attribute: ...
Additional IMAP capabilities: ?.
Security Considerations: ...
4. Security Considerations
<< Danger of an automatic initiation of an action based on $Junk
keyword >>
5. Known issues with mail clients
A popular open source mail client Mozilla implemented "labels". Each
label has associated color and name (description) that can be changed
by a user, a label name can be stored on an IMAP server as a keyword.
Mozilla uses 5 hardcoded keywords $Label1, $Label2, $Label3, $Label4
and $Label5. However, there is a problem with how Mozilla
implemented this feature. Each label has a fixed IMAP keyword and a
description that can be changed. Now imagine that two users (or even
the same user on two different computers) configured Mozilla
differently so that the label $Label1 has description "Important" on
one machine and description "Junk" on another. As label description
is not stored on the IMAP server there is an interoperability
problem.
There are two possible ways to fix the problem.
One way is to disallow editing of label descriptions (the defaults
are Important/Work/Personal/ToDo/Later) and to use matching keywords
from this document (or create new ones if necessary).
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Another way is to allow users to edit keyword names in addition to
label descriptions. This is a feature for advanced users and should
have a detailed documentation describing how to correctly select
keyword names.
6. Formal Syntax
The following syntax specification uses the augmented Backus-Naur
Form (BNF) notation as specified in [RFC-822] as modified by [IMAP4].
Non-terminals referenced but not defined below are as defined by
[IMAP4].
Except as noted otherwise, all alphabetic characters are case-
insensitive. The use of upper or lower case characters to define
token strings is for editorial clarity only. Implementations MUST
accept these strings in a case-insensitive fashion.
flag_keyword ::= spe_keywords / other_keywords
spe_keywords ::= "$" ( "Forwarded" / "Important" / "ShouldReply" /
/ "Work" / "Personal" ) / junk_keywords
junk_keywords ::= (junkauto_keys / junkmanual_keys)
junkauto_keys ::= "$AutoJunk" / "$AutoNotJunk" / "$AutoMaybeJunk"
junkmanual_keys ::= "$Junk" / "$NotJunk"
other_keywords ::= atom
7. Acknowledgments
The creation of this document was prompted by one of many discussions
on the IMAP mailing list.
John Neystadt co-authored the first revision of this document.
Special thanks to Chris Newman, David Harris, Lyndon Nerenberg and
Mark Crispin for reviewing the document.
The author would also like to thank the developers of Mozilla mail
clients for providing food for thoughts.
8. References
[KEYWORDS] Bradner, S., "Key words for use in RFCs to Indicate
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Requirement Levels", RFC 2119, March 1997.
[IMAP4] Crispin, M., "Internet Message Access Protocol - Version
4rev1", RFC 3501, University of Washington, March 2003.
[RFC 2822] Resnick, P., "Internet Message Format", RFC 2822, QUALCOMM
Incorporated, April 2001.
[] Palme, J., "Common Internet Message Headers", RFC 2076,
Stockholm University/KTH, February 1997.
[ANNOTATION] Gellens, R., Daboo, C., "IMAP ANNOTATE Extension", work
in progress, <http://www.ietf.org/internet-drafts/draft-ietf-imapext-
annotate-xx.txt>
9. Author's Addresses
Alexey Melnikov
Isode Limited
Address: 5 Castle Business Village,
36 Station Road,
Hampton, Middlesex,
United Kingdom, TW12 2BX
Phone: +44 77 53759732
Email: mel@isode.com
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10. Full Copyright Statement
Copyright (C) The Internet Society (2002-2003). All Rights Reserved.
This document and translations of it may be copied and furnished to
others, and derivative works that comment on or otherwise explain it
or assist in its implementation may be prepared, copied, published
and distributed, in whole or in part, without restriction of any
kind, provided that the above copyright notice and this paragraph are
included on all such copies and derivative works. However, this
document itself may not be modified in any way, such as by removing
the copyright notice or references to the Internet Society or other
Internet organizations, except as needed for the purpose of
developing Internet standards in which case the procedures for
copyrights defined in the Internet Standards process must be
followed, or as required to translate it into languages other than
English.
The limited permissions granted above are perpetual and will not be
revoked by the Internet Society or its successors or assigns.
This document and the information contained herein is provided on an
"AS IS" basis and THE INTERNET SOCIETY AND THE INTERNET ENGINEERING
TASK FORCE DISCLAIMS 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.
Acknowledgement
Funding for the RFC Editor function is currently provided by the
Internet Society.
Appendix A. To do list and open issues
Open issues are enclosed in << and >> through out this document.
This document is still very raw. Comments are encouraged.
<< Define a regular template?:
Purpose (description): ...
Private or Shared on a server: private/shared/both
Is it an advisory keyword or may it cause an automatic action?
When/by whom the keyword is set/cleared: ...
Notes: (e.g. - limitations, mutually exclusive with ...)
How the flag should be mapped to ANNOTATE (especially for mutually
exclusive flags)
Additional IMAP capability strings if the server is required to do
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special processing ...
Security Considerations: ...
>>
<<Define special capabilities returned by cooperating servers?>>
<<Register "$" prefix for user defined keywords with special
semantics?>>
<<Add mutually exclusive $Important/$Unimportant/$NormalImportance?>>
<<Replace $ShouldReply with a more generic $ToDo?>>
<<Mozilla uses the following labels:
Important/Work/Personal/ToDo/Later. Labels in Mozilla are mutually
exclusive (i.e. at least one or none). I personally disagree that
Important is mutually exclusive with either Work or Personal. Same
about ToDo. I am not sure what is the purpose of the "Later" label>>
Appendix B. Change History
Changes from draft-melnikov-imap-keywords-01
1. Dropped $Adult as it was too controversial.
2. Replaced $Spam with $Junk/$NotJunk/$AutoJunk/$AutoNotJunk and
$AutoMaybeJunk. Reworked text to reflect this and tried to
catch all different interactions between newly introduced
keywords.
3. Added more usage examples for different keywords.
4. Updated IMAP4rev1 reference
5. Added description of a problem with Mozilla.
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