Sieve Email Filtering: Extension for Notifications
RFC 5435

Network Working Group                                   A. Melnikov, Ed.
Request for Comments: 5435                                 Isode Limited
Category: Standards Track                                  B. Leiba, Ed.
                                                            W. Segmuller
                                         IBM T.J. Watson Research Center
                                                               T. Martin
                                                       Endless Crossword
                                                            January 2009

           Sieve Email Filtering: Extension for Notifications

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.

Copyright Notice

   Copyright (c) 2009 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.

Abstract

   Users go to great lengths to be notified as quickly as possible that
   they have received new mail.  Most of these methods involve polling
   to check for new messages periodically.  A push method handled by the
   final delivery agent gives users quicker notifications and saves
   server resources.  This document does not specify the notification
   method, but it is expected that using existing instant messaging
   infrastructure such as Extensible Messaging and Presence Protocol
   (XMPP), or Global System for Mobile Communications (GSM) Short
   Message Service (SMS) messages will be popular.  This document
   describes an extension to the Sieve mail filtering language that
   allows users to give specific rules for how and when notifications
   should be sent.

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Table of Contents

   1. Introduction ....................................................3
      1.1. Conventions Used in This Document ..........................3
   2. Capability Identifier ...........................................3
   3. Notify Action ...................................................3
      3.1. Notify Action Syntax and Semantics .........................3
      3.2. Notify Parameter "method" ..................................3
      3.3. Notify Tag ":from" .........................................4
      3.4. Notify Tag ":importance" ...................................4
      3.5. Notify Tag ":options" ......................................5
      3.6. Notify Tag ":message" ......................................5
      3.7. Examples ...................................................6
      3.8. Requirements on Notification Methods Specifications ........7
   4. Test valid_notify_method ........................................8
   5. Test notify_method_capability ...................................9
   6. Modifier encodeurl to the 'set' Action .........................10
   7. Interactions with Other Sieve Actions ..........................11
   8. Security Considerations ........................................11
   9. IANA Considerations ............................................13
      9.1. Registration of Sieve Extension ...........................13
      9.2. New Registry for Sieve Notification Mechanisms ............14
      9.3. New Registry for Notification-Capability Parameters .......14
   10. Acknowledgements ..............................................15
   11. References ....................................................16
      11.1. Normative References .....................................16
      11.2. Informative References ...................................16

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1.  Introduction

   This is an extension to the Sieve language defined by [Sieve] for
   providing instant notifications.  It defines the new action "notify".

   This document does not specify the notification methods.  Examples of
   possible notification methods are email and XMPP.  To allow for the
   portability of scripts that use notifications, implementation of the
   [MailTo] method is mandatory.  Other available methods shall depend
   upon the implementation and configuration of the system.

1.1.  Conventions Used in This Document

   Conventions for notations are as in [Sieve], Section 1.1, including
   the use of [ABNF].

   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 [Kwds].

2.  Capability Identifier

   The capability string associated with the extension defined in this
   document is "enotify".

3.  Notify Action

3.1.  Notify Action Syntax and Semantics

   Usage:  notify [":from" string]
           [":importance" <"1" / "2" / "3">]
           [":options" string-list]
           [":message" string]
           <method: string>

   The "notify" action specifies that a notification should be sent to a
   user.  The format of the notification is implementation-defined and
   is also affected by the notification method used (see Section 3.2).
   However, all content specified in the ":message" parameter SHOULD be
   included.

3.2.  Notify Parameter "method"

   The "method" positional parameter identifies the notification method
   that will be used; it is a URI [URI].  For example, the notification
   method can be a tel URI [TEL-URI] with a phone number to send SMS
   messages to, or an XMPP [XMPP] URI containing an XMPP identifier
   [XMPP-URI].

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   The supported URI values will be site-specific, but support for the
   [MailTo] method is REQUIRED in order to ensure interoperability.  If
   a URI schema is specified that the implementation does not support,
   the notification MUST cause an error condition at run time.  Sieve
   scripts can check the supported methods using the valid_notify_method
   test to be sure that they only use supported ones, to avoid such
   error conditions.

   If the "method" parameter contains a supported URI schema, then the
   URI MUST be checked for syntactic validity.  Invalid URI syntax or an
   unsupported URI extension MUST cause an error.  An implementation MAY
   enforce other semantic restrictions on URIs -- for example, to
   restrict phone numbers in a tel: URI to a particular geographical
   region -- and will treat violations of such semantic restrictions as
   errors.

3.3.  Notify Tag ":from"

   A ":from" tag may be used to specify an author of the notification.
   The syntax of this parameter's value is method-specific.
   Implementations SHOULD check the syntax according to the notification
   method specification and generate an error when a syntactically
   invalid ":from" tag is specified.

   In order to minimize/prevent forgery of the author value,
   implementations SHOULD impose restrictions on what values can be
   specified in a ":from" tag.  For example, an implementation may
   restrict this value to be a member of a list of known author
   addresses or to belong to a particular domain.  It is suggested that
   values that don't satisfy such restrictions simply be ignored rather
   than causing the "notify" action to fail.

3.4.  Notify Tag ":importance"

   The ":importance" tag specifies the importance of quick delivery of
   the notification, as perceived by the Sieve script owner.  The
   ":importance" tag is followed by a numeric value represented as a
   string: "1" (high importance), "2" (normal importance), and "3" (low
   importance).  If no importance is given, the default value "2" SHOULD
   be assumed.  A notification method MAY treat the importance value as
   a transport indicator.  For example, it might deliver notifications
   of high importance quicker than notifications of normal or low
   importance.  Some notification methods allow users to specify their
   state of activity (for example, "busy" or "away from keyboard").  If
   the notification method provides this information, it SHOULD be used
   to selectively send notifications.  If, for example, the user marks

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   herself as "busy", a notification method can require that a
   notification with importance of "3" is not to be sent; however, the
   user might be notified of a notification with higher importance.

   If the notification method allows users to filter messages based upon
   certain parameters in the message, users SHOULD be able to filter
   based upon importance.  If the notification method does not support
   importance, then this parameter MUST be ignored.  An implementation
   MAY include the importance value in the default message, Section 3.6,
   if one is not provided.

3.5.  Notify Tag ":options"

   The ":options" tag is used to send additional parameters to the
   notification method.  Interpretation of the parameters is method-
   specific.  This document doesn't specify any such additional
   parameter.

   Each string in the options string list has the following syntax:
   "<optionname>=<value>"
   where optionname has the following ABNF [ABNF]:

      l-d = ALPHA / DIGIT
      l-d-p = l-d / "." / "-" / "_"
      optionname = l-d *l-d-p
      value = *(%x01-09 / %x0B-0C / %x0E-FF)

3.6.  Notify Tag ":message"

   The ":message" tag specifies the message data to be included in the
   notification.  The entirety of the string SHOULD be sent, but
   implementations MAY shorten the message for technical or aesthetic
   reasons.  If the ":message" parameter is absent, a default
   implementation-specific message is used.  Unless otherwise specified
   by a particular notification mechanism, an implementation default
   containing at least the value of the "From" header field and the
   value of the "Subject" header field is RECOMMENDED.

   In order to construct more complex messages, the notify extension can
   be used together with the Sieve variables extension [Variables], as
   shown in the examples below.

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3.7.  Examples

   Example 1:
       require ["enotify", "fileinto", "variables"];

       if header :contains "from" "boss@example.org" {
           notify :importance "1"
               :message "This is probably very important"
                           "mailto:alm@example.com";
           # Don't send any further notifications
           stop;
       }

       if header :contains "to" "sievemailinglist@example.org" {
           # :matches is used to get the value of the Subject header
           if header :matches "Subject" "*" {
               set "subject" "${1}";
           }

           # :matches is used to get the value of the From header
           if header :matches "From" "*" {
               set "from" "${1}";
           }

           notify :importance "3"
               :message "[SIEVE] ${from}: ${subject}"
               "mailto:alm@example.com";
           fileinto "INBOX.sieve";
       }

   Example 2:
       require ["enotify", "fileinto", "variables", "envelope"];

       if header :matches "from" "*@*.example.org" {
           # :matches is used to get the MAIL FROM address
           if envelope :all :matches "from" "*" {
               set "env_from" " [really: ${1}]";
           }

           # :matches is used to get the value of the Subject header
           if header :matches "Subject" "*" {
               set "subject" "${1}";
           }

           # :matches is used to get the address from the From header
           if address :matches :all "from" "*" {
               set "from_addr" "${1}";
           }

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           notify :message "${from_addr}${env_from}: ${subject}"
                           "mailto:alm@example.com";
       }

 Example 3:
     require ["enotify", "variables"];

     set "notif_method"
     "xmpp:tim@example.com?message;subject=SIEVE;body=You%20got%20mail";

     if header :contains "subject" "Your dog" {
         set "notif_method" "tel:+14085551212";
     }

     if header :contains "to" "sievemailinglist@example.org" {
         set "notif_method" "";
     }

     if not string :is "${notif_method}" "" {
         notify "${notif_method}";
     }

     if header :contains "from" "boss@example.org" {
         # :matches is used to get the value of the Subject header
         if header :matches "Subject" "*" {
             set "subject" "${1}";
         }

         # don't need high importance notification for
         # a 'for your information'
         if not header :contains "subject" "FYI:" {
             notify :importance "1" :message "BOSS: ${subject}"
                                "tel:+14085551212";
         }
     }

3.8.  Requirements on Notification Methods Specifications

   This section describes requirements for documents that define
   specific Sieve notification methods.

   Notification mechanisms MUST NOT add new Sieve tags to the "notify"
   action.

   A notification method MAY allow modification of the final
   notification text -- for example, truncating it if it exceeds a
   length limit or modifying characters that can not be represented in
   the target character set.  Characters in the notification text that

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   can't be represented by the notification method SHOULD be replaced
   with a symbol indicating an unknown character.  Allowed modifications
   MUST be documented in the document describing the notification
   method.

   A notification method MAY ignore parameters specified in the "notify"
   action.

   A notification method MAY recommend the default message value to be
   used if the ":message" argument is not specified.

   Notifications SHOULD include timestamps, if the notification method
   allows for their transmission outside of the textual message.
   Implementation methods that can only transmit timestamps in the
   textual message MAY include them in the textual message.

   A notification MUST include means to identify/track its origin in
   order to allow a recipient to stop notifications or find out how to
   contact the sender.  This requirement is to help with tracking a
   misconfigured or abusive origin of notifications.

   Methods SHOULD NOT include any other extraneous information not
   specified in parameters to the "notify" action.

   Methods MUST specify which URI parameters (if any) must be ignored,
   which ones must be used in the resulting notification, and which ones
   must cause an error.

   Methods MUST specify what values are returned by the
   notify_method_capability test, Section 5, in particular for the
   "online" notification-capability.

   If there are errors sending the notification, the Sieve interpreter
   SHOULD ignore the notification and not retry indefinitely.  The Sieve
   interpreter MAY throttle notifications; if it does, a request to send
   a notification MAY be silently ignored.  Documents describing
   notification methods SHOULD describe how retries, throttling,
   duplicate suppression (if any), etc. are to be handled by
   implementations.

4.  Test valid_notify_method

   Usage:  valid_notify_method <notification-uris: string-list>

   The valid_notify_method test is true if the notification methods
   listed in the notification-uris argument are supported and they are
   valid both syntactically (including URI parameters) and semantically

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   (including implementation-specific semantic restrictions).  This test
   MUST perform exactly the same validation as would be performed on the
   "method" parameter to the "notify" action.

   The test is true only if ALL of the listed notification methods are
   supported and valid.

   Example 4 (partial):
             if not valid_notify_method ["mailto:",
                     "http://gw.example.net/notify?test"] {
                 stop;
             }

5.  Test notify_method_capability

   Usage:  notify_method_capability [COMPARATOR] [MATCH-TYPE]
           <notification-uri: string>
           <notification-capability: string>
           <key-list: string-list>

   The notify_method_capability test retrieves the notification
   capability specified by the notification-capability string that is
   specific to the notification-uri and matches it to the values
   specified in the key-list.  The test succeeds if a match occurs.  The
   type of match defaults to ":is", and the default comparator is
   "i;ascii-casemap".

   The notification-capability parameter is case insensitive.

   The notify_method_capability test MUST fail unconditionally if the
   specified notification-uri is syntactically invalid (as determined by
   the valid_notify_method test, Section 4) or specifies an unsupported
   notification method.  However this MUST NOT cause an error.

   The notify_method_capability test MUST fail unconditionally if the
   specified notification-capability item is not known to the Sieve
   interpreter.  A script MUST NOT fail with an error if the item does
   not exist.  This allows scripts to be written that handle nonexistent
   items gracefully.

   This document defines a single notification-capability value
   "online", which is described below.  Additional notification-
   capability values may be defined by using the procedure defined in
   Section 9.3.

   The "relational" extension [Relational] adds a match type called
   ":count".  The count of an notify_method_capability test is 0, if the
   returned information is the empty string, or 1.

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   For the "online" notification-capability, the
   notify_method_capability test can match one of the following key-list
   values:

   o  "yes" - the entity identified by the notification-uri can receive
      a notify notification immediately.  Note that even after this
      value is returned, there is no guarantee that the entity would
      actually be able to receive any notification immediately or even
      receive it at all.  Transport errors, recipient policy, etc. can
      prevent that.

   o  "no" - the entity identified by the notification-uri is not
      currently available to receive an immediate notification.

   o  "maybe" - the Sieve interpreter can't determine if the entity
      identified by the notification-uri is online or not.

   Example 5:
             require ["enotify"];

             if notify_method_capability
                    "xmpp:tim@example.com?message;subject=SIEVE"
                    "Online"
                    "yes" {
                 notify :importance "1" :message "You got mail"
                      "xmpp:tim@example.com?message;subject=SIEVE";
             } else {
                 notify :message "You got mail" "tel:+14085551212";
             }

6.  Modifier encodeurl to the 'set' Action

   Usage:  ":encodeurl"

   When the Sieve script specifies both "variables" [Variables] and
   "enotify" capabilities in the "require", a new "set" action modifier
   (see [Variables]) ":encodeurl" becomes available to Sieve scripts.
   This modifier performs percent-encoding of any octet in the string
   that doesn't belong to the "unreserved" set (see [URI]).  The
   percent-encoding procedure is described in [URI].

   The ":encodeurl" modifier has precedence 15.

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   Example 6:
       require ["enotify", "variables"];

       set :encodeurl "body_param" "Safe body&evil=evilbody";

       notify "mailto:tim@example.com?body=${body_param}";

7.  Interactions with Other Sieve Actions

   The "notify" action is compatible with all other actions, and does
   not affect the operation of other actions.  In particular, the
   "notify" action MUST NOT cancel the implicit keep.

   Multiple executed "notify" actions are allowed.  Specific
   notification methods MAY allow multiple notifications from the same
   script to be collapsed into one.

8.  Security Considerations

   Security considerations are discussed in [Sieve].  Additionally,
   implementations must be careful to follow the security considerations
   of the specific notification methods.

   The "notify" action is potentially very dangerous.  The path the
   notification takes through the network may not be secure.  An error
   in the options string may cause the message to be transmitted to
   someone it was not intended for, or may expose information to
   eavesdroppers.

   Just because a notification is received doesn't mean that it was sent
   by the Sieve implementation.  It might be possible to forge
   notifications or modify parts of valid notifications with some
   notification methods.

   Forgery of the ":importance" value (for example, by unauthorized
   script modification) can potentially result in slowdown in
   notification delivery.

   Note that some components of notifications should not be trusted.
   For example, the timestamp field can be easily forged or modified
   when some notification transports are used.  Even if the timestamp is
   believed to be correct by the sender and is not modified in transit,
   it might be misleading on the receiving system due to clock
   differences.

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   An organization may have a policy about the forwarding of classified
   information to unclassified networks.  Unless the policy is also
   enforced in the module responsible for the generating (or sending) of
   notifications, users can use the extension defined in this document
   to extract classified information and bypass the policy.

   Notifications can result in loops and bounces.  Also, allowing a
   single script to notify multiple destinations can be used as a means
   of amplifying the number of messages in an attack.  Moreover, if loop
   detection is not properly implemented, it may be possible to set up
   exponentially growing notification loops.  Accordingly, Sieve
   notification methods:

   1.  MUST provide mechanisms for avoiding notification loops.

   2.  MUST provide the means for administrators to limit the ability of
       users to abuse notify.  In particular, it MUST be possible to
       limit the number of "notify" actions a script can perform.
       Additionally, if no use cases exist for using "notify" with
       multiple destinations, this limit SHOULD be set to 1.  Additional
       limits, such as the ability to restrict "notify" to local users,
       MAY also be implemented.

   3.  MUST provide facilities to log the use of "notify" in order to
       facilitate tracking down abuse.

   4.  MAY use script analysis to determine whether or not a given
       script can be executed safely.  While the Sieve language is
       sufficiently complex so that full analysis of all possible
       scripts is computationally infeasible, the majority of real-world
       scripts are amenable to analysis.  For example, an implementation
       might allow scripts that it has determined to be safe to run
       unhindered, block scripts that are potentially problematic, and
       subject unclassifiable scripts to additional auditing and
       logging.

   Allowing "notify" action at all may not be appropriate in situations
   where Sieve scripts are associated with email accounts that are
   freely-available and/or not trackable to a human who can be held
   accountable for creating message bombs or other abuse.

   Implementations that construct URIs internally from various notify
   parameters MUST make sure that all components of such URIs are
   properly percent-encoded (see [URI]).  In particular, this applies to
   values of the ":from" and ":message" tagged arguments and may apply
   to the ":options" values.

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   Header/envelope tests [Sieve], together with Sieve variables, can be
   used to extract the list of users to receive notifications from the
   incoming email message or its envelope.  This is potentially quite
   dangerous, as this can be used for denial-of-service attacks on
   recipients controlled by the message sender.  For this reason,
   implementations SHOULD NOT allow the use of variables containing
   values extracted from the email message in the "method" parameter to
   the "notify" action.  Note that violation of this SHOULD NOT may
   result in the creation of an open relay, i.e., any sender would be
   able to create specially crafted email messages that would result in
   notifications delivered to recipients under the control of the
   sender.  In the worst case, this might result in financial loss by
   the user controlling the Sieve script and/or by recipients of
   notifications (e.g., if a notification is an SMS message).

   Note that the last SHOULD NOT is not a generic prohibition of use of
   variables in the "notify" action, as controlling the target of a
   notification by extracting it from user-owned data stores (such as
   user's Lightweight Directory Access Protocol (LDAP) entry) is
   considered to be useful.

   It is imperative that whatever implementations use to store the user-
   defined filtering scripts protect them from unauthorized
   modification, to preserve the integrity of the mail system.  An
   attacker who can modify a script can cause mail to be discarded,
   rejected, or forwarded to an unauthorized recipient.  In addition,
   it's possible that Sieve scripts might expose private information,
   such as mailbox names or email addresses of favored (or disfavored)
   correspondents.  Because of that, scripts SHOULD also be protected
   from unauthorized retrieval.

9.  IANA Considerations

9.1.  Registration of Sieve Extension

   To:  iana@iana.org
   Subject:  Registration of new Sieve extension
   Capability name:  enotify
   Description:  adds the "notify" action for notifying user about the
      received message.  It also provides two new tests:
         valid_notify_method checks notification URIs for validity;
         notify_method_capability can check recipients capabilities.
   RFC number:  this RFC
   Contact address:  The Sieve discussion list
      <ietf-mta-filters@imc.org>

   This information has been added to the list of Sieve extensions
   available from http://www.iana.org/.

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9.2.  New Registry for Sieve Notification Mechanisms

   IANA has created a new registry for Sieve notification mechanisms.
   This registry contains both vendor-controlled notification mechanism
   names (beginning with "vnd.") and IETF-controlled notification
   mechanism names.  Vendor-controlled notification mechanism names have
   the format as defined in the following paragraph and may be
   registered on a "First Come First Served" basis [IANA-GUIDELINES], by
   applying to IANA with the form specified later in this section.
   Registration of notification mechanisms that do not begin with "vnd."
   are registered using a "Specification Required" policy
   [IANA-GUIDELINES].

   Vendor-controlled notification mechanism names MUST have the form
   "vnd.<vendor-name>.<mechanism-name>", where <vendor-name> is as
   specified in the Application Configuration Access Protocol (ACAP)
   Vendor Subtree registry [ACAP].

   This defines the template for a new registry for Sieve notification
   mechanisms, which has been created and is available from
   http://www.iana.org/.  There are no initial entries for this
   registry.

   To:  iana@iana.org
   Subject:  Registration of new Sieve notification mechanism
   Mechanism name:  [the name of the mechanism]
   Mechanism URI:  [the RFC number of the document that defines the URI
      used by this mechanism.  Different mechanisms MUST use different
      URI schema.]
   Mechanism-specific options:  [the names of any Sieve notify options
      (as used in the ":options" parameter) that are specific to this
      mechanism, or "none"]
   Permanent and readily available reference:  [the RFC number or an URL
      of the document that defines this notification mechanism]
   Person and email address to contact for further information:  [the
      name and email address of the technical contact for information
      about this mechanism]

9.3.  New Registry for Notification-Capability Parameters

   IANA has created a new registry for the notification-capability
   parameters of the notify_method_capability test.  This registry
   contains both vendor-controlled notification-capability values
   (beginning with "vnd.") and IETF-controlled notification-capability
   values.  Vendor-controlled notification-capability values have the
   format as defined in the following paragraph and may be registered on
   a "First Come First Served" basis [IANA-GUIDELINES], by applying to
   IANA with the form specified later in this section.  Registration of

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   notification-capability values that do not begin with "vnd." are
   registered using the "Specification Required" policy
   [IANA-GUIDELINES].

   Vendor-controlled notification-capability values MUST have the form
   "vnd.<vendor-name>.<capability-name>", where <vendor-name> is as
   specified in the ACAP Vendor Subtree registry [ACAP].

   The following template must be used for registering notification-
   capability parameters:

   To:  iana@iana.org
   Subject:  Registration of a new notification-capability parameter
   Capability name:  [the name of the notification-capability]
   Description:  [an explanation of the purpose of the notification-
      capability]
   Syntax:  [formal definition of allowed values and their syntax]
   Permanent and readily available reference(s):  [the RFC number(s) or
      an URL of the document that defines this notification mechanism]
   Contact information:  [the name and email address of the technical
      contact for information about this mechanism]

   Below is the registration form for the "online" notification-
   capability:

   To:  iana@iana.org
   Subject:  Registration of a new notification-capability parameter
   Capability name:  online
   Description:  Returns whether the entity identified by the
      notification-uri parameter to the notify_method_capability test
      can receive a notify notification immediately.
   Syntax:  Can contain one of three values: "yes", "no", and, "maybe".
      Values MUST be in lowercase.
   Permanent and readily available reference(s):  This RFC
   Contact information:  The Sieve discussion list
      <ietf-mta-filters@imc.org>

10.  Acknowledgements

   Thanks to Larry Greenfield, Sarah Robeson, Tim Showalter, Cyrus
   Daboo, Nigel Swinson, Kjetil Torgrim Homme, Michael Haardt, Mark E.
   Mallett, Ned Freed, Lisa Dusseault, Dilyan Palauzov, Arnt
   Gulbrandsen, Peter Saint-Andre, Sean Turner, Cullen Jennings, and
   Pasi Eronen for help with this document.

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11.  References

11.1.  Normative References

   [ABNF]             Crocker, D., Ed. and P. Overell, "Augmented BNF
                      for Syntax Specifications: ABNF", STD 68,
                      RFC 5234, January 2008.

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

   [MailTo]           Leiba, B. and M. Haardt, "Sieve Notification
                      Mechanism: mailto", RFC 5436, January 2009.

   [Relational]       Segmuller, W. and B. Leiba, "Sieve Extension:
                      Relational Tests", RFC 5231, January 2008.

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

   [URI]              Berners-Lee, T., Fielding, R., and L. Masinter,
                      "Uniform Resource Identifier (URI): Generic
                      Syntax", STD 66, RFC 3986, January 2005.

   [Variables]        Homme, K., "Sieve Extension: Variables", RFC 5229,
                      January 2008.

11.2.  Informative References

   [ACAP]             Newman, C. and J. Myers, "ACAP -- Application
                      Configuration Access Protocol", RFC 2244,
                      November 1997.

   [IANA-GUIDELINES]  Narten, T. and H. Alvestrand, "Guidelines for
                      Writing an IANA Considerations Section in RFCs",
                      BCP 26, RFC 5226, May 2008.

   [TEL-URI]          Schulzrinne, H., "The tel URI for Telephone
                      Numbers", RFC 3966, December 2004.

   [XMPP]             Saint-Andre, Ed., P., "Extensible Messaging and
                      Presence Protocol (XMPP): Core", RFC 3920,
                      October 2004.

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   [XMPP-URI]         Saint-Andre, P., "Internationalized Resource
                      Identifiers (IRIs) and Uniform Resource
                      Identifiers (URIs) for the Extensible Messaging
                      and Presence Protocol (XMPP)", RFC 5122,
                      February 2008.

Authors' Addresses

   Alexey Melnikov (editor)
   Isode Limited
   5 Castle Business Village
   36 Station Road
   Hampton, Middlesex  TW12 2BX
   UK

   EMail: Alexey.Melnikov@isode.com

   Barry Leiba (editor)
   IBM T.J. Watson Research Center
   19 Skyline Drive
   Hawthorne, NY  10532
   US

   Phone: +1 914 784 7941
   EMail: leiba@watson.ibm.com

   Wolfgang Segmuller
   IBM T.J. Watson Research Center
   19 Skyline Drive
   Hawthorne, NY  10532
   US

   Phone: +1 914 784 7408
   EMail: werewolf@us.ibm.com

   Tim Martin
   Endless Crossword
   672 Haight st.
   San Francisco, CA  94117
   US

   Phone: +1 510 260-4175
   EMail: timmartin@alumni.cmu.edu

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