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rfc:rfc5232

Network Working Group A. Melnikov Request for Comments: 5232 Isode Limited Category: Standards Track January 2008

            Sieve Email Filtering: Imap4flags 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

 Recent discussions have shown that it is desirable to set different
 IMAP (RFC 3501) flags on message delivery.  This can be done, for
 example, by a Sieve interpreter that works as a part of a Mail
 Delivery Agent.
 This document describes an extension to the Sieve mail filtering
 language for setting IMAP flags.  The extension allows setting of
 both IMAP system flags and IMAP keywords.

Table of Contents

 1. Introduction ....................................................2
    1.1. Conventions Used ...........................................2
 2. General Requirements for Flag Handling ..........................3
 3. Actions .........................................................3
    3.1. Action setflag .............................................4
    3.2. Action addflag .............................................4
    3.3. Action removeflag ..........................................5
 4. Test hasflag ....................................................6
 5. Tagged Argument :flags ..........................................7
 6. Interaction with Other Sieve Actions ............................8
 7. Security Considerations .........................................8
 8. IANA Considerations .............................................8
 9. Extended Example ................................................8
 10. Acknowledgments ...............................................10
 11. Normative References ..........................................10

Melnikov Standards Track [Page 1] RFC 5232 Sieve: Imap4flags Extension January 2008

1. Introduction

 This is an extension to the Sieve language defined by [SIEVE] for
 setting [IMAP] flags.  It adds a new tagged argument to "keep" and
 "fileinto" that describes the list of flags that have to be set when
 the message is delivered to the specified mailbox.  It also adds
 several actions to help manipulate list of flags and a test to check
 whether a flag belongs to a list.
 From the user's perspective, this extension provides several
 capabilities.  First, it allows manipulation of sets of IMAP flags.
 Second, it gives the ability to associate a set of IMAP flags with a
 message that is delivered to a mailstore using the keep/fileinto
 actions.  Third, it maintains an internal variable.  The internal
 variable contains the default flags that will be associated with a
 message that is delivered using the keep, implicit keep, or fileinto
 actions, when the :flags tagged argument is not specified.  When the
 Sieve [VARIABLES] extension is also supported by the Sieve engine, it
 enables support for multiple variables containing sets of IMAP flags.
 The capability string associated with the extension defined in this
 document is "imap4flags".  All conformant implementations MUST
 implement all Sieve actions (setflag, addflag, removeflag), the
 "hasflag" test, and the ":flags" tagged argument described in this
 document.
 The "imap4flags" extension can be used with or without the
 "variables" extension [VARIABLES].  When the "variables" extension is
 enabled in a script using <require "variables">, the script can use
 explicit variable names in setflag/addflag/removeflag actions and the
 hasflag test.  See also Section 3 for more details.  When the
 "variables" extension is not enabled, the explicit variable name
 parameter to setflag/addflag/removeflag/hasflag MUST NOT be used and
 MUST cause an error according to [SIEVE].

1.1. Conventions Used

 Conventions for notations are as in [SIEVE], Section 1.1, including
 use of "Usage:" label for the definition of action and tagged
 arguments syntax.
 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.

Melnikov Standards Track [Page 2] RFC 5232 Sieve: Imap4flags Extension January 2008

2. General Requirements for Flag Handling

 The following notes apply to processing of addflag/removeflag/setflag
 actions, the "hasflag" test and the ":flags" tagged argument.
 A Sieve interpreter MUST ignore empty strings (i.e., "") in a list-
 of-flags parameter.
 A string containing a space-separated list of flag names is
 equivalent to a string list consisting of the flags.  This
 requirement is to simplify amalgamation of multiple flag lists.
 The Sieve interpreter SHOULD check the list of flags for validity as
 described by [IMAP] ABNF.  In particular, according to [IMAP], non-
 ASCII characters are not allowed in flag names.  However, spaces MUST
 always be allowed as delimiters in strings representing a list of
 flags.  In such strings, multiple spaces between flag names MUST be
 treated as a single space character, and leading and trailing spaces
 MUST be ignored.
 If a flag validity check fails, the flag MUST be ignored.
 Note that it is not possible to use this extension to set or clear
 the \Recent flag or any other special system flag that is not
 settable in [IMAP].  Any such flags MUST be ignored if included in a
 flag list.

3. Actions

 All actions described in this specification (setflag, addflag,
 removeflag) operate on string variables that contain a set of [IMAP]
 flags.  On variable substitution, a set of flags is represented as a
 string containing a space-separated list of flag names.
 Any setflag/addflag/removeflag action MAY alter the flag list in any
 way that leaves its semantics as a set of case-insensitive words
 unaltered.  For example, it may reorder the flags, alter the case of
 the letters in them, or add or remove duplicates or extraneous
 spaces.  Scripts MUST NOT make assumptions about the ordering of
 flags in lists or the preservation of their case.
 Note that the parameter specifying a variable name to
 setflag/addflag/removeflag actions and the hasflag test is optional.
 If the parameter is not specified, the actions operate on the
 internal variable, which has the empty value when the script starts
 execution.  If the SIEVE interpreter doesn't support the "variables"
 extension [VARIABLES], the presence of the variable name parameter
 will cause a runtime error [SIEVE].

Melnikov Standards Track [Page 3] RFC 5232 Sieve: Imap4flags Extension January 2008

 The "addflag" action adds flags to an existing set.  The "removeflag"
 action removes flags from an existing set.  The "setflag" action
 replaces an existing set of flags with a new set.  The "set" action
 defined in [VARIABLES] can be used to replace an existing set of
 flags with a new set as well.  However, it should be noted that the
 "set" action can't perform any flag reordering, duplicate
 elimination, etc.
 The :flags tagged argument to "keep" and "fileinto" actions is used
 to associate a set of flags with the current message.  If the :flags
 tagged argument is not specified with those two actions, the current
 value of the internal variable is used instead.  The value of the
 internal variable also applies to the implicit keep.
 Note that when keep/fileinto is used multiple times in a script and
 duplicate message elimination is performed (as described in Section
 2.10.3 of [SIEVE]), the last flag list value MUST win.

3.1. Action setflag

 Usage:   setflag [<variablename: string>]
          <list-of-flags: string-list>
 Setflag is used for setting [IMAP] system flags or keywords.
 Setflag replaces any previously set flags.
 Example:  if size :over 500K {
               setflag "\\Deleted";
           }
 A more substantial example is the following:
 Example:
      if header :contains "from" "boss@frobnitzm.example.edu" {
          setflag "flagvar" "\\Flagged";
          fileinto :flags "${flagvar}" "INBOX.From Boss";
      }

3.2. Action addflag

 Usage:   addflag [<variablename: string>]
          <list-of-flags: string-list>
 Addflag is used to add flags to a list of [IMAP] flags.  It doesn't
 replace any previously set flags.  This means that multiple
 occurrences of addflag are treated additively.

Melnikov Standards Track [Page 4] RFC 5232 Sieve: Imap4flags Extension January 2008

 The following examples demonstrate requirements described in Section
 2.1.  The following two actions
    addflag "flagvar" "\\Deleted";
    addflag "flagvar" "\\Answered";
 produce the same result as the single action
    addflag "flagvar" ["\\Deleted", "\\Answered"];
 or
    addflag "flagvar" "\\Deleted \\Answered";
 or
    addflag "flagvar" "\\Answered \\Deleted";

3.3. Action removeflag

 Usage:   removeflag [<variablename: string>]
          <list-of-flags: string-list>
 Removeflag is used to remove flags from a list of [IMAP] flags.
 Removeflag clears flags previously set by "set"/"addflag".  Calling
 removeflag with a flag that wasn't set before is not an error and is
 ignored.  Note that if an implementation doesn't perform automatic
 duplicate elimination, it MUST remove all occurrences of the flags
 specified in the second parameter to removeflag.  Empty strings in
 the list-of-flags MUST be ignored.  Also note that flag names are
 case-insensitive, as described in [IMAP].  Multiple removeflag
 actions are treated additively.
    Example:
      if header :contains "Disposition-Notification-To"
         "mel@example.com" {
          addflag "flagvar" "$MDNRequired";
      }
      if header :contains "from" "imap@cac.washington.example.edu" {
          removeflag "flagvar" "$MDNRequired";
          fileinto :flags "${flagvar}" "INBOX.imap-list";
      }

Melnikov Standards Track [Page 5] RFC 5232 Sieve: Imap4flags Extension January 2008

4. Test hasflag

 Usage: hasflag [MATCH-TYPE] [COMPARATOR]
        [<variable-list: string-list>]
        <list-of-flags: string-list>
 The hasflag test evaluates to true if any of the variables matches
 any flag name.  The type of match defaults to ":is".  If the list of
 variables is omitted, value of the internal variable is used instead.
 The default comparator is "i;ascii-casemap", which is the same case-
 insensitive comparison as defined for IMAP flags by [IMAP].
 The "relational" extension [RELATIONAL] adds a match type called
 ":count".  The :count of a variable returns the number of distinct
 flags in it.  The count of a list of variables is the sum of the
 counts of the member variables.
 Example:
   If the internal variable has the value "A B", the following test
    hasflag :is "b A"
   evaluates to true.  The above test can also be written as
    hasflag ["b","A"]
 Example:
   If the variable "MyVar" has value "NonJunk Junk gnus-forward
   $Forwarded NotJunk JunkRecorded $Junk $NotJunk", the following
   tests evaluate to true:
    hasflag :contains "MyVar" "Junk"
    hasflag :contains "MyVar" "forward"
    hasflag :contains "MyVar" ["label", "forward"]
    hasflag :contains "MyVar" ["junk", "forward"]
   Note that the last of these tests can be rewritten
   as
    hasflag :contains "MyVar" "junk forward"
   or
    hasflag :contains "MyVar" "forward junk"
   However, the last two forms are not recommended.

Melnikov Standards Track [Page 6] RFC 5232 Sieve: Imap4flags Extension January 2008

   And the following tests will evaluate to false:
    hasflag :contains "MyVar" "label"
    hasflag :contains "MyVar" ["label1", "label2"]
 Example:
   If the variable "MyFlags" has the value "A B", the following test
     hasflag :count "ge" :comparator "i;ascii-numeric"
             "MyFlags" "2"
   evaluates to true, as the variable contains 2 distinct flags.

5. Tagged Argument :flags

 This specification adds a new optional tagged argument ":flags" that
 alters the behavior of actions "keep" and "fileinto".
 The :flags tagged argument specifies that the flags provided in the
 subsequent argument should be set when fileinto/keep delivers the
 message to the target mailbox/user's main mailbox.  If the :flags
 tagged argument is not specified, "keep" or "fileinto" will use the
 current value of the internal variable when delivering the message to
 the target mailbox.
 Usage:   ":flags" <list-of-flags: string-list>
 The copy of the message filed into the mailbox will have only flags
 listed after the :flags tagged argument.
 The Sieve interpreter MUST ignore all flags that it can't store
 permanently.  This means that the interpreter MUST NOT treat failure
 to store any flag as a runtime failure to execute the Sieve script.
 For example, if the mailbox "INBOX.From Boss" can't store any flags,
 then
   fileinto :flags "\\Deleted" "INBOX.From Boss";
 and
   fileinto "INBOX.From Boss";
 are equivalent.
 This document doesn't dictate how the Sieve interpreter will set the
 [IMAP] flags.  In particular, the Sieve interpreter may work as an
 IMAP client or may have direct access to the mailstore.

Melnikov Standards Track [Page 7] RFC 5232 Sieve: Imap4flags Extension January 2008

6. Interaction with Other Sieve Actions

 This extension works only on the message that is currently being
 processed by Sieve; it doesn't affect another message generated as a
 side effect of any action or any other message already in the
 mailstore.
 The extension described in this document doesn't change the implicit
 keep (see Section 2.10.2 of [SIEVE]).

7. Security Considerations

 Security considerations are discussed in [IMAP], [SIEVE], and
 [VARIABLES].
 This extension intentionally doesn't allow setting [IMAP] flags on an
 arbitrary message in the [IMAP] message store.
 It is believed that this extension doesn't introduce any additional
 security concerns.

8. IANA Considerations

 The following template specifies the IANA registration of the
 variables Sieve extension specified in this document:
 To: iana@iana.org
 Subject: Registration of new Sieve extension
 Capability name: imap4flags
 Description:     Adds actions and tests for manipulating IMAP flags
 RFC number:      RFC 5232
 Contact address: The Sieve discussion list <ietf-mta-filters@imc.org>
 This information has been added to the list of Sieve extensions given
 on http://www.iana.org/assignments/sieve-extensions.

9. Extended Example

 #
 # Example Sieve Filter
 # Declare any optional features or extension used by the script
 #
 require ["fileinto", "imap4flags", "variables"];
 #
 # Move large messages to a special mailbox
 #

Melnikov Standards Track [Page 8] RFC 5232 Sieve: Imap4flags Extension January 2008

 if size :over 1M
         {
         addflag "MyFlags" "Big";
         if header :is "From" "boss@company.example.com"
                    {
 # The message will be marked as "\Flagged Big" when filed into
 # mailbox "Big messages"
                    addflag "MyFlags" "\\Flagged";
                    }
         fileinto :flags "${MyFlags}" "Big messages";
         }
 if header :is "From" "grandma@example.net"
         {
         addflag "MyFlags" ["\\Answered", "$MDNSent"];
 # If the message is bigger than 1Mb it will be marked as
 # "Big \Answered $MDNSent" when filed into mailbox "grandma".
 # If the message is shorter than 1Mb it will be marked as
 # "\Answered $MDNSent"
         fileinto :flags "${MyFlags}" "GrandMa";
         }
 #
 # Handle messages from known mailing lists
 # Move messages from IETF filter discussion list to filter folder
 #
 if header :is "Sender" "owner-ietf-mta-filters@example.org"
         {
         set "MyFlags" "\\Flagged $Work";
 # Message will have both "\Flagged" and $Work flags
         keep :flags "${MyFlags}";
         }
 #
 # Keep all messages to or from people in my company
 #
 elsif anyof address :domain :is ["From", "To"] "company.example.com"
         {
         keep :flags "${MyFlags}"; # keep in "Inbox" folder
         }
 # Try to catch unsolicited email.  If a message is not to me,
 # or it contains a subject known to be spam, file it away.
 #
 elsif anyof (not address :all :contains
                ["To", "Cc"] "me@company.example.com",
              header :matches "subject"
                ["*make*money*fast*", "*university*dipl*mas*"])

Melnikov Standards Track [Page 9] RFC 5232 Sieve: Imap4flags Extension January 2008

         {
         remove "MyFlags" "\\Flagged";
         fileinto :flags "${MyFlags}" "spam";
         }
 else
         {
         # Move all other external mail to "personal"
         # folder.
         fileinto :flags "${MyFlags}" "personal";
         }

10. Acknowledgments

 This document has been revised in part based on comments and
 discussions that took place on and off the Sieve mailing list.
 The help of those who took the time to review the document and make
 suggestions is appreciated, especially that of Tim Showalter, Barry
 Leiba, Randall Gellens, Ken Murchison, Cyrus Daboo, Matthew Elvey,
 Jutta Degener, Ned Freed, Marc Mutz, Nigel Swinson, Kjetil Torgrim
 Homme, Mark E.  Mallett, Dave Cridland, Arnt Gulbrandsen, Philip
 Guenther, Rob Austein, Sam Hartman, Eric Gray, and Cullen Jennings.
 Special thanks to Tony Hansen and David Lamb for helping me better
 explain the concept.

11. Normative References

 [SIEVE]      Guenther, P., Ed., and T. Showalter, Ed., "Sieve: An
              Email Filtering Language", RFC 5228, January 2008.
 [IMAP]       Crispin, M., "INTERNET MESSAGE ACCESS PROTOCOL - VERSION
              4rev1", RFC 3501, March 2003.
 [VARIABLES]  Homme, K., "Sieve Email Filtering: Variables Extension",
              RFC 5229, January 2008.
 [RELATIONAL] Segmuller, W. and B. Leiba "Sieve Email Filtering:
              Relational Extension", RFC 5231, January 2008.

Melnikov Standards Track [Page 10] RFC 5232 Sieve: Imap4flags Extension January 2008

Author's Address

 Alexey Melnikov
 Isode Limited
 5 Castle Business Village
 Hampton, Middlesex
 United Kingdom, TW12 2BX
 EMail: alexey.melnikov@isode.com

Melnikov Standards Track [Page 11] RFC 5232 Sieve: Imap4flags Extension January 2008

Full Copyright Statement

 Copyright (C) The IETF Trust (2008).
 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, THE IETF TRUST 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.

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Melnikov Standards Track [Page 12]

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