Network Working Group T. Hansen

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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RFC 5703 Sieve MIME Operations October 2009

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" "[email protected]"
{
fileinto "INBOX.part-from-tim";
}

In this example, any message that contains a MIME Content-From header
at the top-level matching the text "[email protected]" 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" "[email protected]"
{
# :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: [email protected]
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
<[email protected]>.

12.2. mime capability

To: [email protected]
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
<[email protected]>.

12.3. replace capability

To: [email protected]
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
<[email protected]>.

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12.4. enclose capability

To: [email protected]
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
<[email protected]>.

12.5. extracttext capability

To: [email protected]
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
<[email protected]>.

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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[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.

Hansen & Daboo Standards Track [Page 17]

RFC 5703 Sieve MIME Operations October 2009

Authors' Addresses

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

EMail: [email protected]

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

EMail: [email protected]
URI: http://www.apple.com/

Hansen & Daboo Standards Track [Page 18]