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PROPOSED STANDARD

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








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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.







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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].



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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.




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   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";
        }









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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.




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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.



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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
   #



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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*"])



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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.












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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









































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RFC 5232              Sieve: Imap4flags Extension           January 2008


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