Network Working Group                                           S. Bosch
Internet-Draft                                          January 16, 2013
Intended status: Standards Track
Expires: July 20, 2013

         Sieve Email Filtering: Detecting Duplicate Deliveries


   This document defines a new test command "duplicate" for the "Sieve"
   email filtering language.  It can be used to test whether a
   particular string value is a duplicate, i.e. whether it was seen
   before by the delivery agent that is executing the Sieve script.  The
   main application for this new test is detecting duplicate message
   deliveries commonly caused by mailing list subscriptions or
   redirected mail addresses.

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   the Trust Legal Provisions and are provided without warranty as
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Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . . . 3
   2.  Conventions Used in This Document . . . . . . . . . . . . . . . 3
   3.  Test "duplicate"  . . . . . . . . . . . . . . . . . . . . . . . 3
   4.  Sieve Capability Strings  . . . . . . . . . . . . . . . . . . . 5
   5.  Examples  . . . . . . . . . . . . . . . . . . . . . . . . . . . 5
   6.  Security Considerations . . . . . . . . . . . . . . . . . . . . 6
   7.  IANA Considerations . . . . . . . . . . . . . . . . . . . . . . 6
   8.  References  . . . . . . . . . . . . . . . . . . . . . . . . . . 7
     8.1.  Normative References  . . . . . . . . . . . . . . . . . . . 7
     8.2.  Informative References  . . . . . . . . . . . . . . . . . . 7
   Author's Address  . . . . . . . . . . . . . . . . . . . . . . . . . 8

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1.  Introduction

   This is an extension to the Sieve filtering language defined by RFC
   5228 [SIEVE].  It adds a test to determine whether a certain string
   value was seen before by the delivery agent in an earlier execution
   of the Sieve script.  This can be used to detect and handle duplicate
   message deliveries.

   Duplicate deliveries are a common side-effect of being subscribed to
   a mailing list.  For example, if a member of the list decides to
   reply to both the user and the mailing list itself, the user will get
   one copy of the message directly and another through mailing list.
   Also, if someone cross-posts over several mailing lists to which the
   user is subscribed, the user will receive a copy from each of those
   lists.  In another scenario, the user has several redirected mail
   addresses all pointing to his main mail account.  If one of the
   user's contacts sends the message to more than one of those
   addresses, the user will receive more than a single copy.  Using the
   "duplicate" extension, users have the means to detect and handle such
   duplicates, e.g. by discarding them, marking them as "seen", or
   putting them in a special folder.

   Duplicate messages are normally detected using the Message-ID header
   field, which is required to be unique for each message.  However, the
   "duplicate" test is flexible enough to use different (weaker)
   criteria for defining what makes a message a duplicate, for example
   based on the subject line.  Also, other applications of this new test
   command are possible, as long as the tracked value is a string.

2.  Conventions Used in This Document

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   document are to be interpreted as described in [KEYWORDS].

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

3.  Test "duplicate"

   Usage: "duplicate" [":handle" <handle: string>]
                      [":header" <header-name: string> /
                          ":value" <value: string>]
                      [":seconds" <timeout: number>]

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   The "duplicate" test keeps track of which values were seen before by
   this test in an earlier execution of this Sieve script.  In its basic
   form, the tested value is the content of the Message-ID header of the
   message.  This way, this test can be used to detect duplicate
   deliveries of the same message.  It can also detect duplicate
   deliveries based on other message header fields if requested and it
   can even use a user-provided string value, e.g. as composed from text
   extracted from the message using the "variables" [VARIABLES]

   The "duplicate" test evaluates to "true" when the provided value was
   seen before in an earlier Sieve execution for a previous message
   delivery.  If the value was not seen earlier, the test evaluates to

   As a side-effect, the "duplicate" test adds the evaluated value to an
   internal duplicate tracking list, so that the test will evaluate to
   "true" the next time the Sieve script is executed and the same value
   is encountered.  Note that the "duplicate" test MUST only check for
   duplicates amongst values encountered in previous executions of the
   Sieve script; it MUST NOT consider values encountered earlier in the
   current Sieve script execution as potential duplicates.  This means
   that all "duplicate" tests in a Sieve script execution, including
   those located in scripts included using the "include" [INCLUDE]
   extension, MUST yield the same result if the arguments are identical.

   Implementations MUST prevent adding values to the internal duplicate
   tracking list when the Sieve script execution fails.  For example,
   this can be implemented by deferring the definitive modification of
   the tracking list to the end of the Sieve script execution.  If
   failed script executions would add values to the duplicate tracking
   list, all "duplicate" tests would erroneously yield "true" for the
   next delivery attempt of the same message, which can -- depending on
   the action taken for a duplicate -- easily lead to discarding the
   message without further notice.

   Implementations SHOULD limit the number of values (and thereby
   messages) that are tracked.  Also, implementations SHOULD let entries
   in the value tracking list expire after a short period of time.  The
   user can explicitly control the length of this expiration time by
   means of the ":seconds" argument.  If the ":seconds" argument is
   omitted, an appropriate default MUST be used.  Sites SHOULD impose a
   maximum limit on the expiration time.  If that limit is exceeded, the
   maximum value MUST silently be substituted; exceeding the limit MUST
   NOT produce an error.

   By default, the tracked value is the content of the message's
   Message-ID header field.  For more advanced purposes, the content of

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   another header can be chosen for tracking by specifying the ":header"
   argument.  The tracked string value can also be specified explicitly
   using the ":value" argument.  The ":header" and ":value" arguments
   are mutually exclusive and specifying both for a single "duplicate"
   test command MUST trigger an error at compile time.  If the value is
   extracted from a header, i.e. when the ":value" argument is not used,
   leading and trailing whitespace (see Section 2.2 of RFC 5228 [SIEVE])
   MUST first be trimmed from the value before performing the actual
   duplicate verification.

   Using the ":handle" argument, the duplicate test can be employed for
   multiple independent purposes.  Only when the tracked value was seen
   before in an earlier script execution by a "duplicate" test with the
   same ":handle" argument, it is recognized as a duplicate.

   NOTE: The necessary mechanism to track duplicate messages is very
   similar to the mechanism that is needed for tracking duplicate
   responses for the "vacation" [VACATION] action.  One way to implement
   the necessary mechanism for the "duplicate" test is therefore to
   store a hash of the tracked value and, if provided, the ":handle"

4.  Sieve Capability Strings

   A Sieve implementation that defines the "duplicate" test command will
   advertise the capability string "duplicate".

5.  Examples

   In the following basic example message duplicates are detected by
   tracking the Message-ID header.  Duplicate deliveries are stored in a
   special folder contained in the user's Trash folder.  If the folder
   does not exist, it is created automatically using the "mailbox"
   [MAILBOX] extension.  This way, the user has a chance to recover
   messages when necessary.  Messages that are not recognized as
   duplicates are stored in the user's inbox as normal.

   require ["duplicate", "fileinto", "mailbox"];

   if duplicate {
     fileinto :create "Trash/Duplicate";

   The next example shows a more complex use of the "duplicate" test.
   The user gets network alerts from a set of remote automated
   monitoring systems.  Multiple notifications can be received about the

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   same event from different monitoring systems.  The Message-ID of
   these messages is different, because these are all distinct messages
   from different senders.  To avoid being notified multiple times about
   the same event the user writes the following script:

   require ["duplicate", "variables", "imap4flags",

   if header :matches "subject" "ALERT: *" {
     if duplicate :seconds 60 :value "${1}" {
       setflag "\\seen";
     fileinto "Alerts";

   The subjects of the notification message are structured with a
   predictable pattern which includes a description of the event.  In
   the script above the "duplicate" test is used to detect duplicate
   alert events.  The message subject is matched against a pattern and
   the event description is extracted using the "variables" [VARIABLES]
   extension.  If a message with that event in the subject was received
   before, but more than a minute ago, it is not detected as a duplicate
   due to the specified ":seconds" argument.  In the the event of a
   duplicate, the message is marked as "seen" using the "imap4flags"
   [IMAP4FLAGS] extension.  All alert messages are put into the "Alerts"
   mailbox irrespective of whether those messages are duplicates or not.

6.  Security Considerations

   A flood of unique messages could cause the list of tracked values to
   grow indefinitely.  Implementations therefore SHOULD implement limits
   on the number and lifespan of entries in that list.

7.  IANA Considerations

   The following template specifies the IANA registration of the Sieve
   extension specified in this document:

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      Subject: Registration of new Sieve extension

      Capability name: duplicate
      Description:     Adds test 'duplicate' that can be used to test
                       whether a particular string value is a duplicate,
                       i.e. whether it was seen before by the delivery
                       agent that is executing the Sieve script. The
                       main application for this test is detecting
                       duplicate message deliveries.
      RFC number:      this RFC
      Contact address: Sieve mailing list <>

   This information should be added to the list of sieve extensions
   given on

8.  References

8.1.  Normative References

   [INCLUDE]  Daboo, C. and A. Stone, "Sieve Email Filtering: Include
              Extension", RFC 6609, May 2012.

              Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119, March 1997.

   [SIEVE]    Guenther, P. and T. Showalter, "Sieve: An Email Filtering
              Language", RFC 5228, January 2008.

8.2.  Informative References

              Melnikov, A., "Sieve Email Filtering: Imap4flags
              Extension", RFC 5232, January 2008.

   [MAILBOX]  Melnikov, A., "The Sieve Mail-Filtering Language --
              Extensions for Checking Mailbox Status and Accessing
              Mailbox Metadata", RFC 5490, March 2009.

              Showalter, T. and N. Freed, "Sieve Email Filtering:
              Vacation Extension", RFC 5230, January 2008.

              Homme, K., "Sieve Email Filtering: Variables Extension",
              RFC 5229, January 2008.

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Author's Address

   Stephan Bosch


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