Network Working Group K. Murchison

Network Working Group K. Murchison
Request for Comments: 5233 Carnegie Mellon University
Obsoletes: 3598 January 2008
Category: Standards Track

Sieve Email Filtering: Subaddress Extension

Status of This Memo

This document specifies an Internet standards track protocol for the
Internet community, and requests discussion and suggestions for
improvements. Please refer to the current edition of the "Internet
Official Protocol Standards" (STD 1) for the standardization state
and status of this protocol. Distribution of this memo is unlimited.

Abstract

On email systems that allow for 'subaddressing' or 'detailed
addressing' (e.g., "[email protected]"), it is sometimes
desirable to make comparisons against these sub-parts of addresses.
This document defines an extension to the Sieve Email Filtering
Language that allows users to compare against the user and detail
sub-parts of an address.

Table of Contents

1. Introduction ....................................................2
2. Conventions Used in This Document ...............................2
3. Capability Identifier ...........................................2
4. Subaddress Comparisons ..........................................2
5. IANA Considerations .............................................5
6. Security Considerations .........................................5
7. Normative References ............................................5
Appendix A. Acknowledgments ........................................6
Appendix B. Changes since RFC 3598 .................................6

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1. Introduction

Subaddressing is the practice of augmenting the local-part of an
[RFC2822] address with some 'detail' information in order to give
some extra meaning to that address. One common way of encoding
'detail' information into the local-part is to add a 'separator
character sequence', such as "+", to form a boundary between the
'user' (original local-part) and 'detail' sub-parts of the address,
much like the "@" character forms the boundary between the local-part
and domain.

Typical uses of subaddressing might be:

o A message addressed to "[email protected]" is delivered into a
mailbox called "sieve" belonging to the user "ken".

o A message addressed to "5551212#[email protected]" is delivered to
the voice mailbox number "123" at phone number "5551212".

This document describes an extension to the Sieve language defined by
[RFC5228] for comparing against the 'user' and 'detail' sub-parts of
an address.

2. Conventions Used in This Document

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. Capability Identifier

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

4. Subaddress Comparisons

Test commands that act exclusively on addresses may take the optional
tagged arguments ":user" and ":detail" to specify what sub-part of
the local-part of the address will be acted upon.

NOTE: In most cases, the envelope "to" address is the preferred
address to examine for subaddress information when the desire is
to sort messages based on how they were addressed so as to get to
a specific recipient. The envelope address is, after all, the
reason a given message is being processed by a given sieve script
for a given user. This is particularly true when mailing lists,

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RFC 5233 Sieve: Subaddress Extension January 2008

aliases, and 'virtual domains' are involved since the envelope may
be the only source of detail information for the specific
recipient.

NOTE: Because the encoding of detailed addresses are site and/or
implementation specific, using the subaddress extension on foreign
addresses (such as the envelope "from" address or originator
header fields) may lead to inconsistent or incorrect results.

The ":user" argument specifies the user sub-part of the local-part of
an address. If the address is not encoded to contain a detail sub-
part, then ":user" specifies the entire left side of the address
(equivalent to ":localpart").

The ":detail" argument specifies the detail sub-part of the local-
part of an address. If the address is not encoded to contain a
detail sub-part, then the address fails to match any of the specified
keys. If a zero-length string is encoded as the detail sub-part,
then ":detail" resolves to the empty value ("").

NOTE: If the encoding method used for detailed addresses utilizes
a separator character sequence, and the separator character
sequence occurs more than once in the local-part, then the logic
used to split the address is implementation-defined and is usually
dependent on the format used by the encompassing mail system.

Implementations MUST make sure that the encoding method used for
detailed addresses matches that which is used and/or allowed by the
encompassing mail system, otherwise unexpected results might occur.
Note that the mechanisms used to define and/or query the encoding
method used by the mail system are outside the scope of this
document.

The ":user" and ":detail" address parts are subject to the same rules
and restrictions as the standard address parts defined in [RFC5228],
Section 2.7.4.

For convenience, the "ADDRESS-PART" syntax element defined in
[RFC5228], Section 2.7.4, is augmented here as follows:

ADDRESS-PART =/ ":user" / ":detail"

A diagram showing the ADDRESS-PARTs of an email address where the
detail information follows a separator character sequence of "+" is
shown below:

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:user "+" :detail "@" :domain
\-----------------/
:local-part

A diagram showing the ADDRESS-PARTs of a email address where the
detail information precedes a separator character sequence of "--" is
shown below:

:detail "--" :user "@" :domain
\------------------/
:local-part

Example (where the detail information follows "+"):

require ["envelope", "subaddress", "fileinto"];

# In this example the same user account receives mail for both
# "[email protected]" and "[email protected]"

# File all messages to postmaster into a single mailbox,
# ignoring the :detail part.
if envelope :user "to" "postmaster" {
fileinto "inbox.postmaster";
stop;
}

# File mailing list messages (subscribed as "ken+mta-filters").
if envelope :detail "to" "mta-filters" {
fileinto "inbox.ietf-mta-filters";
}

# Redirect all mail sent to "ken+foo".
if envelope :detail "to" "foo" {
redirect "[email protected]";
}

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RFC 5233 Sieve: Subaddress Extension January 2008

5. IANA Considerations

The following template specifies the IANA registration of the
subaddress Sieve extension specified in this document. This
registration replaces that from RFC 3598:

To: [email protected]
Subject: Registration of new Sieve extension

Capability name: subaddress
Description: Adds the ':user' and ':detail' address parts
for use with the address and envelope tests
RFC number: RFC 5233
Contact address: The Sieve discussion list <[email protected]>

This information has been added to the list of Sieve extensions given
on http://www.iana.org/assignments/sieve-extensions.

6. Security Considerations

Security considerations are discussed in [RFC5228]. It is believed
that this extension does not introduce any additional security
concerns.

7. Normative References

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

[RFC2822] Resnick, P., "Internet Message Format", RFC 2822, April
2001.

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

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RFC 5233 Sieve: Subaddress Extension January 2008

Appendix A. Acknowledgments

Thanks to Tim Showalter, Alexey Melnikov, Michael Salmon, Randall
Gellens, Philip Guenther, Jutta Degener, Michael Haardt, Ned Freed,
Mark Mallett, and Barry Leiba for their help with this document.

Appendix B. Changes since RFC 3598

o Discussion of how the user and detail information is encoded now
uses generic language.

o Added note detailing that this extension is most useful when used
on the envelope "to" address.

o Added note detailing that this extension isn't very useful on
foreign addresses (envelope "from" or originator header fields).

o Fixed envelope test example to only use "to" address.

o Replaced ":user" example with one that doesn't produce unexpected
behavior.

o Refer to the zero-length string ("") as "empty" instead of "null"
(per RFC 5228).

o Use only RFC 2606 domains in examples.

o Miscellaneous editorial changes.

Author's Address

Kenneth Murchison
Carnegie Mellon University
5000 Forbes Avenue
Cyert Hall 285
Pittsburgh, PA 15213
USA

Phone: +1 412 268 2638
EMail: [email protected]

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RFC 5233 Sieve: Subaddress Extension January 2008

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