    Network Working Group                                          T. Hansen
    Request for Comments: 5703                             AT&T Laboratories
    Category: Standards Track                                       C. Daboo
                                                                  Apple Inc.
                                                                October 2009

         Sieve Email Filtering: MIME Part Tests, Iteration, Extraction,
                           Replacement, and Enclosure

    Abstract

       This document defines extensions to the Sieve email filtering
       language to permit analysis and manipulation of the MIME body parts
       of an email message.

    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.  Code Components extracted from this document must
       include Simplified BSD License text as described in Section 4.e of
       the Trust Legal Provisions and are provided without warranty as
       described in the BSD License.

       This document may contain material from IETF Documents or IETF
       Contributions published or made publicly available before November
       10, 2008.  The person(s) controlling the copyright in some of this
       material may not have granted the IETF Trust the right to allow
       modifications of such material outside the IETF Standards Process.
       Without obtaining an adequate license from the person(s) controlling
       the copyright in such materials, this document may not be modified

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       outside the IETF Standards Process, and derivative works of it may
       not be created outside the IETF Standards Process, except to format
       it for publication as an RFC or to translate it into languages other
       than English.

    Table of Contents

       1. Introduction ....................................................2
       2. Conventions Used in This Document ...............................3
       3. Sieve Loops: Actions "foreverypart" and "break" .................3
       4. Changes to Sieve Tests ..........................................4
          4.1. Test "header" ..............................................4
          4.2. Test "address" .............................................7
          4.3. Test "exists" ..............................................8
       5. Action "replace" ................................................8
       6. Action "enclose" ...............................................10
       7. Action "extracttext" ...........................................11
       8. Sieve Capability Strings .......................................11
       9. Examples .......................................................12
          9.1. Example 1 .................................................12
          9.2. Example 2 .................................................12
          9.3. Example 3 .................................................13
       10. Acknowledgements ..............................................13
       11. Security Considerations .......................................14
       12. IANA Considerations ...........................................14
          12.1. foreverypart capability ..................................15
          12.2. mime capability ..........................................15
          12.3. replace capability .......................................15
          12.4. enclose capability .......................................16
          12.5. extracttext capability ...................................16
       13. References ....................................................16
          13.1. Normative References .....................................16
          13.2. Informative References ...................................17

    1.  Introduction

       MIME messages ([RFC2045]) are often complex objects, consisting of
       many parts and sub-parts.  This Sieve ([RFC5228]) extension defines
       mechanisms for performing tests on MIME body parts, looping through
       the MIME body parts, extracting information from a MIME body part,
       changing the contents of a MIME body part, and enclosing the entire
       message within a wrapper.

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    2.  Conventions Used in This Document

       Conventions for notations are as in [RFC5228], Section 1.1.

       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 [RFC2119].

    3.  Sieve Loops: Actions "foreverypart" and "break"

       The base Sieve language has no looping mechanism.  Given that
       messages may contain multiple parts, in order to support filters that
       apply to any and all parts, we introduce a new control command:
       "foreverypart", which is an iterator that walks though every MIME
       part of a message, including nested parts, depth first, and applies
       the commands in the specified block to each of them.  The iterator
       will start with the first MIME part (as its current context) and will
       execute a command block (Sieve commands enclosed by {...}).  Upon
       completion of this command block, the iterator advances to the next
       MIME part (as its current context) and executes the same command
       block again.

       The iterator can be terminated prematurely by a new Sieve control
       command, "break".

       Usage:  foreverypart [":name" string] block

       Usage:  break [":name" string];

       "foreverypart" commands can be nested inside other "foreverypart"
       commands.  When this occurs, the nested "foreverypart" iterates over
       the MIME parts contained within the MIME part currently being
       targeted by the nearest enclosing "foreverypart" command.  (That is,
       the inner loop only operates on children of the bodypart currently
       accessed by the outer loop.)  If that MIME part is a terminal MIME
       part (i.e., does not contain other MIME parts), then the nested
       "foreverypart" loop is simply ignored.

       Sieve implementations MAY limit the number of nested loops that occur
       within one another; however, they MUST support at least one nested
       loop inside another loop.

       If a name is given to a "break" command, it terminates the closest
       enclosing loop with the identical matching name.  (If a nested
       "foreverypart" name is the same as a "foreverypart" name in an outer
       level, the outer level name is hidden.)  It is an error if there is
       no enclosing loop with that name.

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       If no name is given in a "break" command (i.e., the ":name" parameter
       is omitted), the break command terminates the closest enclosing loop.

    4.  Changes to Sieve Tests

       This specification extends the base Sieve "header", "address", and
       "exists" tests to support targeting those tests at a specific MIME
       part or at all MIME parts in the enclosing scope.

    4.1.  Test "header"

       The "header" test is extended with the addition of new ":mime" and
       ":anychild" tagged arguments and their associated options.

       Usage:  header [":mime"] [":anychild"] [MIMEOPTS]
          [COMPARATOR] [MATCH-TYPE]
          <header-names: string-list> <key-list: string-list>

          The definition of [MIMEOPTS] is:

       Syntax:  ":type" / ":subtype" / ":contenttype" /
          ":param" <param-list: string-list>

       When the ":mime" tagged argument is present in the "header" test, it
       will parse the MIME header lines in the message so that tests can be
       performed on specific elements.  The ":anychild" tagged argument may
       only appear when the ":mime" tagged argument is present, and only
       modifies the semantics of the ":mime" tagged argument.  That is,
       presence of the ":anychild" in absence of ":mime" is an error.

       When used outside the context of a "foreverypart" iterator, and
       without an ":anychild" tagged argument, the "header" test will
       examine only the outer top-level [RFC5322] headers of the message.

       When used inside the context of a "foreverypart" iterator, and
       without an ":anychild" tagged argument, the "header" test will
       examine the headers associated with the current MIME part context
       from the loop.

       When used outside the context of a "foreverypart" iterator, and with
       an ":anychild" tagged argument, the "header" test will examine all
       MIME body parts and return true if any of them satisfies the test.

       When used inside the context of a "foreverypart" iterator, and with
       an ":anychild" tagged argument, the "header" test will examine the
       current MIME part context and all its nested MIME body parts,
       returning true if any of them satisfies the test.

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       The "header" test with the ":mime" tagged argument can test various
       aspects of certain structured MIME headers.  Implementations SHOULD
       support desegmentation, decoding, and charset translation of
       parameter values encoded according to [RFC2231] as part of this test.
       Additionally, [RFC2047] describes a process whereby [RFC5322] headers
       can be encoded in various ways.  That encoding is not strictly
       allowed in MIME parameters; however, in practice, it has been used in
       many email implementations.  So, Sieve implementations MAY decode
       [RFC2047]-encoded words in parameter values as part of this test.

       These options are available:

       :type          for a "Content-Type" MIME header field, parses and
                      tests the value of the MIME type specified in the
                      header; for a "Content-Disposition" MIME header field,
                      parses and tests the value of the disposition
                      specified in the header; for other MIME headers, uses
                      a blank string for the test.

       :subtype       for a "Content-Type" MIME header field, parses and
                      tests the value of the MIME subtype specified in the
                      header; for a "Content-Disposition" MIME header field,
                      uses a blank string for the test; for other MIME
                      headers, uses a blank string for the test.

       :contenttype   for a "Content-Type" MIME header field, parses and
                      tests the combined value of the MIME type and subtype
                      specified in the header; for a "Content-Disposition"
                      MIME header field, behaves the same as the ":type"
                      option; for other MIME headers, uses a blank string
                      for the test.

       :param         parses the header looking for MIME parameters in the
                      header.  The supplied string-list lists the names of
                      any parameters to be tested.  If any one named
                      parameter value matches any of the test string values,
                      the test will return true.

       When the ":count" option from [RFC5231] is used, the following
       applies:

       a.  for ":type", ":subtype", or ":contenttype", return a count of the
           number of headers that parsed successfully

       b.  for ":param", return a count of the number of parameters with the
           given name that were found

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       Example:

       require ["mime", "fileinto"];

       if header :mime :type "Content-Type" "image"
       {
           fileinto "INBOX.images";
       }

       In this example, any message that contains a MIME image type part at
       the top-level is saved to the mailbox "INBOX.images".

       Example:

       require ["mime", "fileinto"];

       if header :mime :anychild :contenttype
                 "Content-Type" "text/html"
       {
           fileinto "INBOX.html";
       }

       In this example, any message that contains any MIME part with a
       content-type of "text/html" is saved to the mailbox "INBOX.html".

       Example:

       require ["mime", "foreverypart", "fileinto"];

       foreverypart
       {
           if allof (
             header :mime :param "filename" :contains
                "Content-Disposition" "important",
             header :mime :subtype "Content-Type" "pdf",
             size :over "100K")
           {
               fileinto "INBOX.important";
               break;
           }
       }

       In this example, any message that contains a MIME part that has a
       content-disposition with a filename parameter containing the text
       "important", has a content-subtype of "pdf" and is bigger than 100 Kb
       is saved to the mailbox "INBOX.important".

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    4.2.  Test "address"

       The "address" test is extended with the addition of new ":mime" and
       ":anychild" tagged arguments and their associated options.

       Usage:  address [":mime"] [":anychild"] [COMPARATOR]
          [ADDRESS-PART] [MATCH-TYPE]
          <header-list: string-list> <key-list: string-list>

       When the ":mime" tagged argument is present in the "address" test, it
       will parse the MIME header lines as if they were standard address
       header lines in a message so that tests can be performed on specific
       elements.

       The behavior of the ":anychild" tagged argument and the interaction
       with the "foreverypart" iterator is the same as for the extended
       "header" test in Section 4.1.

       That is,

          the use of "address" when both the ":mime" and ":anychild" tagged
          arguments are omitted is the test defined in [RFC5228], i.e., it
          will *only* operate on top-level header fields, whether or not it
          is inside "foreverypart".

          the use of "address" with ":mime" and no ":anychild" operates on
          the current MIME part only (or on the top-level header fields, if
          outside "foreverypart").

          the use of "address" with ":mime" and ":anychild" operates on the
          current MIME part and all of its descendants.

       Example:

       require ["mime", "fileinto"];

       if address :mime :is :all "content-from" "tim@example.com"
       {
           fileinto "INBOX.part-from-tim";
       }

       In this example, any message that contains a MIME Content-From header
       at the top-level matching the text "tim@example.com" is saved to the
       mailbox "INBOX.part-from-tim".

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    4.3.  Test "exists"

       The "exists" test is extended with the addition of the new ":mime"
       and ":anychild" tagged arguments and their associated options.

       Usage:  exists [":mime"] [":anychild"] <header-names: string-list>

       When the ":mime" tagged argument is present in the "exists" test, the
       test is extended to check for the existence of MIME headers in MIME
       parts.

       The behavior of the ":anychild" tagged argument and the interaction
       with the "foreverypart" iterator is the same as for the extended
       "header" test Section 4.1.

       That is,

          the use of "exists" when both the ":mime" and ":anychild" tagged
          arguments are omitted is the test defined in [RFC5228], i.e., it
          will *only* operate on top-level header fields, whether or not it
          is inside "foreverypart".

          the use of "exists" with ":mime" and no ":anychild" operates on
          the current MIME part only (or on the top-level header fields, if
          outside "foreverypart").

          the use of "exists" with ":mime" and ":anychild" operates on the
          current MIME part and all of its descendants.

       Example:

       require ["mime", "fileinto"];

       if exists :mime :anychild "content-md5"
       {
           fileinto "INBOX.md5";
       }

       In this example, any message that contains a MIME Content-MD5 header
       in any MIME part is saved to the mailbox "INBOX.md5".

    5.  Action "replace"

       Usage:  replace [":mime"] [":subject" string] [":from" string]
          <replacement: string>

       The "replace" command is defined to allow a MIME part to be replaced
       with the text supplied in the command.

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       When used in the context of a "foreverypart" iterator, the MIME part
       to be replaced is the "current" MIME part.  If the current MIME
       context is a multipart MIME part, the entire multipart MIME part is
       replaced, which would alter the MIME structure of the message by
       eliminating all of the children of the multipart part.  (Replacing a
       non-multipart MIME part within a "foreverypart" loop context does not
       alter the overall message structure.)  If the MIME structure is
       altered, the change takes effect immediately: the "foreverypart"
       iterator that is executing does not go into the no-longer existing
       body parts, and subsequent "foreverypart" iterators would use the new
       message structure.

       When used outside the context of a "foreverypart" loop, the MIME part
       to be replaced is the entire message.

       If the ":mime" parameter is not specified, the replacement string is
       a text/plain part in UTF-8 [RFC3629].

       If the ":mime" parameter is specified, then the replacement string
       is, in fact, a MIME entity as defined in [RFC2045], Section 2.4,
       including both MIME headers and content.

       If the entire message is being replaced, the optional ":subject"
       parameter specifies a subject line to attach to the message that is
       generated.  UTF-8 characters can be used in the string argument;
       implementations MUST convert the string to [RFC2047]-encoded words if
       and only if non-ASCII characters are present.  If the ":subject"
       parameter is used, implementations MUST preserve any previous Subject
       header as an Original-Subject header.  Implementations MUST preserve
       all other header fields from the original message with the exception
       of those relating to the MIME structure that is being replaced.

       If the entire message is being replaced, as an indication that the
       message is no longer as created by the original author of the
       message, the optional ":from" parameter may be used to specify an
       alternate address to use in the From field of the message that is
       generated.  The string must specify a valid [RFC5322] mailbox-list.
       Implementations SHOULD check the syntax and generate an error when a
       syntactically invalid ":from" parameter is specified.
       Implementations MAY also impose restrictions on what addresses can be
       specified in a ":from" parameter; it is suggested that values that
       fail such a validity check simply be ignored rather than causing the
       "replace" action to fail.  If the From header is changed,
       implementations MUST preserve the previous From header as an
       Original-From header.

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       Implementations that support the "editheader" extension [RFC5293]
       MUST ensure that any Original-Subject or Original-From headers added
       by the system cannot be modified or removed.  Implementations MAY
       prevent the addition of Original-Subject and Orignal-From headers via
       the "editheader" extension.

       If ":mime" is specified and either ":subject" or ":from" is
       specified, the ":subject:" or ":from" parameter MUST be ignored.
       This SHOULD be flagged as a compilation error.

    6.  Action "enclose"

       Usage:  enclose <:subject string> <:headers string-list> string

       A new Sieve action command is defined to allow an entire message to
       be enclosed as an attachment to a new message.  After enclosure,
       subsequent actions affecting the message header or content, as well
       as tests operating on the MIME structure or accessing MIME header
       fields, use the newly created message instead of the original
       message; this means that any use of a "replace" action or other
       similar actions should be executed before the "enclose" action.

       If multiple "enclose" actions are executed by a script, the message
       is enclosed multiple times.  (If a Sieve script desires to choose
       between different enclosures, or wants to delay the enclosure to the
       end of the script, it can use variables with appropriate tests
       [RFC5229].)

       This action does not affect messages that are forwarded via a
       "redirect" action.

       Specifically, the original message becomes a multipart/mixed message
       with two parts: a text/plain portion with the string argument as its
       body, and a message/rfc822 portion with the original message
       enclosed.  The Content-Type: header field becomes multipart/mixed.
       The optional Subject: header is specified by the ":subject" argument;
       if not present, the subject will be taken from the enclosed message.
       Any headers specified by ":headers" are copied from the old message
       into the new message.  If not specified by ":headers", Date: and
       From: headers should be synthesized to reflect the current date and
       the user running the Sieve action.

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    7.  Action "extracttext"

       Usage:  extracttext [MODIFIER] [":first" number] <varname: string>

       The "extracttext" action may be used within the context of a
       "foreverypart" loop and is used to store text into a variable as
       defined by [RFC5229].  Servers MUST support transcoding of any
       textual body part into UTF-8 for use with this action.  This requires
       decoding any transfer encoding as well as transcoding from the
       indicated character set into UTF-8.  It stores at most ":first"
       characters of the transcoded content of the current MIME body part in
       the variable identified by varname.  If the ":first" parameter is not
       present, the whole content of the current MIME body part is stored.
       In either case, the actually stored data MAY be truncated to conform
       to implementation specific limit on variable length and/or on MIME
       body part length.  If the transfer encoding or character set is
       unrecognized by the implementation or recognized but invalid, an
       empty string will result.

       If "extracttext" is used outside the context of a "foreverypart"
       loop, the action will set the variable identified by varname to the
       empty string.  This SHOULD be flagged as a compilation error.

       Modifiers are applied on the extracted text before it is stored in
       the variable.

    8.  Sieve Capability Strings

       A Sieve implementation that defines the "foreverypart" and "break"
       actions will advertise the capability string "foreverypart".

       A Sieve implementation that defines the ":mime" and ":anychild"
       tagged arguments to the "header", "address", and "exists" commands
       will advertise the capability string "mime".

       A Sieve implementation that defines the "replace" action will
       advertise the capability string "replace".

       A Sieve implementation that defines the "enclose" action will
       advertise the capability string "enclose".

       A Sieve implementation that defines the "extracttext" action will
       advertise the capability string "extracttext".  Note that to be
       useful, the "extracttext" action also requires the "variables"
       [RFC5229] and "foreverypart" capabilities.

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    9.  Examples

    9.1.  Example 1

       Consider a Sieve script to replace some of the Windows executable
       attachments in a message.  (The actual list of executable types and
       extensions is considerably longer and constantly changing.  The tests
       shown here are an example only.)  Such a script might look like this:

       require [ "foreverypart", "mime", "replace" ];
       foreverypart
       {
         if anyof (
                header :mime :contenttype :is
                  "Content-Type" "application/exe",
                header :mime :param "filename"
                  :matches ["Content-Type", "Content-Disposition"] "*.com" )
         {
           replace "Executable attachment removed by user filter";
         }
       }

    9.2.  Example 2

       Consider a Sieve script to warn the user about some of the executable
       attachment types.  (The actual list of executable types and
       extensions is considerably longer and constantly changing.  The tests
       shown here are an example only.)  Such a script might look like this:

       require [ "foreverypart", "mime", "enclose" ];

       foreverypart
       {
         if header :mime :param "filename"
            :matches ["Content-Type", "Content-Disposition"]
              ["*.com", "*.exe", "*.vbs", "*.scr",
               "*.pif", "*.hta", "*.bat", "*.zip" ]
         {
           # these attachment types are executable
           enclose :subject "Warning" :text
       WARNING! The enclosed message contains executable attachments.
       These attachment types may contain a computer virus program
       that can infect your computer and potentially damage your data.

       Before clicking on these message attachments, you should verify
       with the sender that this message was sent by them and not a
       computer virus.

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       .
       ;
           break;
         }
       }

    9.3.  Example 3

       A Sieve script to extract subject and text out of messages from the
       boss might look like this:

       require ["mime", "variables", "extracttext"];

       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}";
         }

         # extract the first 100 characters of the first text/* part
         foreverypart
         {
           if header :mime :type :is "Content-Type" "text"
           {
             extracttext :first 100 "msgcontent";
             break;
           }
         }

         # if it's not a 'for your information' message
         if not header :contains "subject" "FYI:"
         {
           # do something using ${subject} and ${msgcontent}
           # such as sending a notification using a
           # notification extension
         }
       }

    10.  Acknowledgements

       Comments from members of the MTA Filters Working Group, in particular
       Ned Freed, Kjetil Torgrim Homme, Mark Mallett, Alexey Melnikov, Aaron
       Stone, and Nigel Swinson are gratefully acknowledged.

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    11.  Security Considerations

       The "enclose" action creates an entirely new message, as compared to
       just redirecting or forwarding the existing message.  Therefore, any
       site policies applicable to message submission should be enforced.

       The looping specification specified here provides easier access to
       information about the message contents, which may also be achieved
       through other sieve tests.  This is not believed to raise any
       additional security issues beyond those for the Sieve "envelope" and
       "body" [RFC5173] tests.

       Any change in message content may interfere with digital signature
       mechanisms that include that content in the signed material.  In
       particular, using "replace" makes direct changes to the body content
       and will affect the body hash included in Domain Keys Identified Mail
       (DKIM) signatures [RFC4871], or the message signature used for Secure
       MIME (S/MIME) [RFC3851], Pretty Good Privacy (PGP) [RFC1991] or
       OpenPGP [RFC4880].

       It is not possible to examine the MIME structure of decrypted content
       in a multipart/encrypted MIME part.

       When "enclose" is used on a message containing a multipart/signed
       MIME part, the Sieve implementation MUST ensure that the original
       message is copied octet-for-octet to maintain the validity of the
       digital signature.

       The system MUST be sized and restricted in such a manner that even
       malicious use of MIME part matching does not deny service to other
       users of the host system.

       All of the security considerations given in the base Sieve
       specification also apply to these extensions.

    12.  IANA Considerations

       The Original-Subject and Original-From headers have been registered
       in the Permanent Message Header Fields registry.

       The following templates specify the IANA registrations of the Sieve
       extensions specified in this document.  This information has been
       added to the IANA registry of Sieve Extensions (currently found at
       http://www.iana.org).

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    12.1.  foreverypart capability

       To: iana@iana.org
       Subject: Registration of new Sieve extension

       Capability name: foreverypart
       Description: adds the "foreverypart" and "break" actions for
       iterating through MIME parts of a message.

       RFC number: RFC 5703
       Contact address: The Sieve discussion list
       <ietf-mta-filters@imc.org>.

    12.2.  mime capability

       To: iana@iana.org
       Subject: Registration of new Sieve extension

       Capability name: mime
       Description: adds the ":mime" and ":anychild" tagged arguments to the
       "header", "address", and "exists" tests.  Adds the ":type",
       ":subtype", ":contenttype", and ":param" options when ":mime" is used
       with the "header" test.

       RFC number: RFC 5703
       Contact address: The Sieve discussion list
       <ietf-mta-filters@imc.org>.

    12.3.  replace capability

       To: iana@iana.org
       Subject: Registration of new Sieve extension

       Capability name: replace
       Description: adds the "replace" action for replacing a MIME body part
       of a message.

       RFC number: RFC 5703
       Contact address: The Sieve discussion list
       <ietf-mta-filters@imc.org>.

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    RFC 5703                 Sieve MIME Operations              October 2009

    12.4.  enclose capability

       To: iana@iana.org
       Subject: Registration of new Sieve extension

       Capability name: enclose
       Description: adds the "enclose" action for enclosing a message with a
       wrapper.

       RFC number: RFC 5703
       Contact address: The Sieve discussion list
       <ietf-mta-filters@imc.org>.

    12.5.  extracttext capability

       To: iana@iana.org
       Subject: Registration of new Sieve extension

       Capability name: extracttext
       Description: adds the "extracttext" action for extracting text from a

       MIME body part.

       RFC number: RFC 5703
       Contact address: The Sieve discussion list
       <ietf-mta-filters@imc.org>.

    13.  References

    13.1.  Normative References

       [RFC2045]  Freed, N. and N. Borenstein, "Multipurpose Internet Mail
                  Extensions (MIME) Part One: Format of Internet Message
                  Bodies", RFC 2045, November 1996.

       [RFC2047]  Moore, K., "MIME (Multipurpose Internet Mail Extensions)
                  Part Three: Message Header Extensions for Non-ASCII Text",
                  RFC 2047, November 1996.

       [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
                  Requirement Levels", BCP 14, RFC 2119, March 1997.

       [RFC2231]  Freed, N. and K. Moore, "MIME Parameter Value and Encoded
                  Word Extensions:
                  Character Sets, Languages, and Continuations", RFC 2231,
                  November 1997.

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    RFC 5703                 Sieve MIME Operations              October 2009

       [RFC3629]  Yergeau, F., "UTF-8, a transformation format of ISO
                  10646", STD 63, RFC 3629, November 2003.

       [RFC5173]  Degener, J. and P. Guenther, "Sieve Email Filtering: Body
                  Extension", RFC 5173, April 2008.

       [RFC5228]  Guenther, P. and T. Showalter, "Sieve: An Email Filtering
                  Language", RFC 5228, January 2008.

       [RFC5229]  Homme, K., "Sieve Email Filtering: Variables Extension",
                  RFC 5229, January 2008.

       [RFC5231]  Segmuller, W. and B. Leiba, "Sieve Email Filtering:
                  Relational Extension", RFC 5231, January 2008.

       [RFC5293]  Degener, J. and P. Guenther, "Sieve Email Filtering:
                  Editheader Extension", RFC 5293, August 2008.

       [RFC5322]  Resnick, P., Ed., "Internet Message Format", RFC 5322,
                  October 2008.

    13.2.  Informative References

       [RFC1991]  Atkins, D., Stallings, W., and P. Zimmermann, "PGP Message
                  Exchange Formats", RFC 1991, August 1996.

       [RFC3851]  Ramsdell, B., "Secure/Multipurpose Internet Mail
                  Extensions (S/MIME) Version 3.1 Message Specification",
                  RFC 3851, July 2004.

       [RFC4871]  Allman, E., Callas, J., Delany, M., Libbey, M., Fenton,
                  J., and M. Thomas, "DomainKeys Identified Mail (DKIM)
                  Signatures", RFC 4871, May 2007.

       [RFC4880]  Callas, J., Donnerhacke, L., Finney, H., Shaw, D., and R.
                  Thayer, "OpenPGP Message Format", RFC 4880, November 2007.

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    RFC 5703                 Sieve MIME Operations              October 2009

    Authors' Addresses

       Tony Hansen
       AT&T Laboratories
       200 Laurel Ave.
       Middletown, NJ  07748
       USA

       EMail: tony+sieveloop@maillennium.att.com

       Cyrus Daboo
       Apple Inc.
       1 Infinite Loop
       Cupertino, CA  95014
       USA

       EMail: cyrus@daboo.name
       URI:   http://www.apple.com/

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