Network Working Group T. Showalter

Network Working Group T. Showalter
Request for Comments: 5230
Category: Standards Track N. Freed, Ed.
Sun Microsystems
January 2008

Sieve Email Filtering: Vacation 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

This document describes an extension to the Sieve email filtering
language for an autoresponder similar to that of the Unix "vacation"
command for replying to messages. Various safety features are
included to prevent problems such as message loops.

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RFC 5230 Sieve: Vacation Extension January 2008

Table of Contents

1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Conventions Used in This Document . . . . . . . . . . . . . . 3
3. Capability Identifier . . . . . . . . . . . . . . . . . . . . 3
4. Vacation Action . . . . . . . . . . . . . . . . . . . . . . . 3
4.1. Days Parameter . . . . . . . . . . . . . . . . . . . . . . 3
4.2. Previous Response Tracking . . . . . . . . . . . . . . . . 4
4.3. Subject and From Parameters . . . . . . . . . . . . . . . 6
4.4. MIME Parameter . . . . . . . . . . . . . . . . . . . . . . 6
4.5. Address Parameter and Limiting Replies to Personal
Messages . . . . . . . . . . . . . . . . . . . . . . . . . 7
4.6. Restricting Replies to Automated Processes and Mailing
Lists . . . . . . . . . . . . . . . . . . . . . . . . . . 8
4.7. Interaction with Other Sieve Actions . . . . . . . . . . . 8
4.8. Examples . . . . . . . . . . . . . . . . . . . . . . . . . 9
5. Response Message Generation . . . . . . . . . . . . . . . . . 9
5.1. SMTP MAIL FROM Address . . . . . . . . . . . . . . . . . . 9
5.2. Date . . . . . . . . . . . . . . . . . . . . . . . . . . . 9
5.3. Subject . . . . . . . . . . . . . . . . . . . . . . . . . 10
5.4. From . . . . . . . . . . . . . . . . . . . . . . . . . . . 10
5.5. To . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10
5.6. Auto-Submitted . . . . . . . . . . . . . . . . . . . . . . 10
5.7. Message Body . . . . . . . . . . . . . . . . . . . . . . . 10
5.8. In-Reply-To and References . . . . . . . . . . . . . . . . 10
6. Relationship to Recommendations for Automatic Responses to
Electronic Mail . . . . . . . . . . . . . . . . . . . . . . . 11
7. Internationalization Considerations . . . . . . . . . . . . . 11
8. Security Considerations . . . . . . . . . . . . . . . . . . . 12
9. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 12
10. References . . . . . . . . . . . . . . . . . . . . . . . . . . 13
10.1. Normative References . . . . . . . . . . . . . . . . . . . 13
10.2. Informative References . . . . . . . . . . . . . . . . . . 13
Appendix A. Acknowledgements . . . . . . . . . . . . . . . . . . 15

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RFC 5230 Sieve: Vacation Extension January 2008

1. Introduction

This document defines an extension to the Sieve language defined in
[RFC5228] for notification that messages to a particular recipient
will not be answered immediately.

2. Conventions Used in This Document

Conventions for notations are as in [RFC5228] section 1.1.

The key words "MUST", "MUST NOT", "SHOULD", "SHOULD NOT", "REQUIRED",
and "MAY" in this document are to be interpreted as defined in
[RFC2119].

3. Capability Identifier

Sieve implementations that implement vacation have an identifier of
"vacation" for use with the capability mechanism.

4. Vacation Action

Usage: vacation [":days" number] [":subject" string]
[":from" string] [":addresses" string-list]
[":mime"] [":handle" string] <reason: string>

The "vacation" action implements a vacation autoresponder similar to
the vacation command available under many versions of Unix. Its
purpose is to provide correspondents with notification that the user
is away for an extended period of time and that they should not
expect quick responses.

"Vacation" is used to respond to a message with another message.
Vacation's messages are always addressed to the Return-Path address
(that is, the envelope from address) of the message being responded
to.

4.1. Days Parameter

The ":days" argument is used to specify the period in which addresses
are kept and are not responded to, and is always specified in days.
The minimum value used for this parameter is normally 1. Sites MAY
define a different minimum value as long as the minimum is greater
than 0. Sites MAY also define a maximum days value, which MUST be
greater than 7, and SHOULD be greater than 30.

If ":days" is omitted, the default value is either 7 or the minimum
value (as defined above), whichever is greater.

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RFC 5230 Sieve: Vacation Extension January 2008

If the parameter given to ":days" is less than the minimum value,
then the minimum value is used instead.

If ":days" exceeds the site-defined maximum, the site-defined maximum
is used instead.

4.2. Previous Response Tracking

"Vacation" keeps track of all the responses it has sent to each
address in some period (as specified by the :days optional argument).
If vacation has not previously sent the response to this address
within the given time period, it sends the "reason" argument to the
SMTP MAIL FROM address [RFC2821] of the message that is being
responded to. (The SMTP MAIL FROM address should be available in the
Return-path: header field if Sieve processing occurs after final
delivery.)

Tracking is not just per address, but must also take the vacation
response itself into account. A script writer might, for example,
have a vacation action that will send a general notice only once in
any two-week period. However, even if a sender has received this
general notice, it may be important to send a specific notice when a
message about something timely or something specific has been
detected.

A particular vacation response can be identified in one of two ways.
The first way is via an explicit :handle argument, which attaches a
name to the response. All vacation statements that use the same
handle will be considered the same response for tracking purposes.

The second way is via a synthesis of the :subject, :from, :mime, and
reason vacation command arguments. All vacation actions that do not
contain an explicit handle and that use an identical combination of
these arguments are considered the same for tracking purposes.

For instance, if [email protected] sends mail to
[email protected] twice, once with the subject "Cyrus bug"
and once with the subject "come over for dinner", and
[email protected] has the script shown below,
[email protected] would receive two responses, one with the
first message, one with the second.

require "vacation";
if header :contains "subject" "cyrus" {
vacation "I'm out -- send mail to cyrus-bugs";
} else {
vacation "I'm out -- call me at +1 304 555 0123";
}

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In the above example, [email protected] gets the second
message despite having gotten the first one because separate vacation
responses have been triggered. This behavior is REQUIRED.

There is one important exception to this rule, however. If the Sieve
variables extension [RFC5229] is used, the arguments MUST NOT have
undergone variable expansion prior to their use in response tracking.
This is so that examples like the following script will only generate
a single response to each incoming message with a different subject
line.

require ["vacation", "variables"];
if header :matches "subject" "*" {
vacation :subject "Automatic response to: ${1}"
"I'm away -- send mail to foo in my absence";
}

As noted above, the optional ":handle" parameter can be used to tell
the Sieve interpreter to treat two vacation actions with different
arguments as the same command for purposes of response tracking. The
argument to ":handle" is a string that identifies the type of
response being sent. For instance, if [email protected] sends
mail to [email protected] twice, one with the subject
"lunch?" and once with the subject "dinner?", and
[email protected] has the script shown below,
[email protected] will only receive a single response. (Which
response is sent depends on the order in which the messages are
processed.)

require "vacation";
if header :contains "subject" "lunch" {
vacation :handle "ran-away" "I'm out and can't meet for lunch";
} else {
vacation :handle "ran-away" "I'm out";
}

NOTE: One way to implement the necessary mechanism here is to store a
hash of either the current handle and the recipient address or, if no
handle is provided, a hash of the vacation action parameters
specifying the message content and the recipient address. If a
script is changed, implementations MAY reset the records of who has
been responded to and when they have been responded to.

IMPLEMENTATION NOTE: Care must be taken in constructing a hash of
vacation action parameters. In particular, since most parameters are
optional, it is important not to let the same string used as the
value for different parameters produce the same hash value. One

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RFC 5230 Sieve: Vacation Extension January 2008

possible way to accomplish this is to apply the hash to a series of
counted or null terminated strings, one for each possible parameter
in particular order.

Implementations are free to limit the number of remembered responses;
however, the limit MUST NOT be less than 1000. When limiting the
number of tracked responses, implementations SHOULD discard the
oldest ones first.

4.3. Subject and From Parameters

The ":subject" parameter specifies a subject line to attach to any
vacation response 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. Implementations MUST generate an appropriate default
subject line as specified below if no :subject parameter is
specified.

A ":from" parameter may be used to specify an alternate address to
use in the From field of vacation messages. The string must specify
a valid [RFC2822] 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 specified in a ":from" parameter; it is
suggested that values that fail such a validity check simply be
ignored rather than cause the vacation action to fail.

4.4. MIME Parameter

The ":mime" parameter, if supplied, specifies that the reason string
is, in fact, a MIME entity as defined in [RFC2045] section 2.4,
including both MIME headers and content.

If the optional :mime parameter is not supplied, the reason string is
considered a UTF-8 string.

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require "vacation";
vacation :mime text:
Content-Type: multipart/alternative; boundary=foo

--foo

I'm at the beach relaxing. Mmmm, surf...

--foo
Content-Type: text/html; charset=us-ascii

<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.0//EN"
"http://www.w3.org/TR/REC-html40/strict.dtd">
<HTML><HEAD><TITLE>How to relax</TITLE>
<BASE HREF="http://home.example.com/pictures/"></HEAD>
<BODY><P>I'm at the <A HREF="beach.gif">beach</A> relaxing.
Mmmm, <A HREF="ocean.gif">surf</A>...
</BODY></HTML>

--foo--
.

4.5. Address Parameter and Limiting Replies to Personal Messages

"Vacation" MUST NOT respond to a message unless the recipient user's
email address is in a "To", "Cc", "Bcc", "Resent-To", "Resent-Cc", or
"Resent-Bcc" line of the original message. An email address is
considered to belong to the recipient if it is one of:

1. an email address known by the implementation to be associated
with the recipient,

2. the final envelope recipient address if it's available to the
implementation, or

3. an address specified by the script writer via the ":addresses"
argument described in the next paragraph.

Users can supply additional mail addresses that are theirs with the
":addresses" argument, which takes a string-list listing additional
addresses that a user might have. These addresses are considered to
belong to the recipient user in addition to the addresses known to
the implementation.

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RFC 5230 Sieve: Vacation Extension January 2008

4.6. Restricting Replies to Automated Processes and Mailing Lists

Implementations MAY refuse to send a vacation response to a message
that contains any header or content that makes it appear that a
response would not be appropriate.

Implementations MUST have a list of addresses that "vacation" MUST
NOT send mail to. However, the contents of this list are
implementation defined. The purpose of this list is to stop mail
from going to addresses used by system daemons that would not care if
the user is actually reading her mail.

Implementations are encouraged, however, to include well-known
addresses like "MAILER-DAEMON", "LISTSERV", "majordomo", and other
addresses typically used only by automated systems. Additionally,
addresses ending in "-request" or beginning in "owner-", i.e.,
reserved for mailing list software, are also suggested.

Implementors may take guidance from [RFC2142], but should be careful.
Some addresses, like "POSTMASTER", are generally actually managed by
people, and people do care if the user is going to be unavailable.

Implementations SHOULD NOT respond to any message that contains a
"List-Id" [RFC2919], "List-Help", "List-Subscribe", "List-
Unsubscribe", "List-Post", "List-Owner", or "List-Archive" [RFC2369]
header field.

Implementations SHOULD NOT respond to any message that has an "Auto-
submitted" header field with a value other than "no". This header
field is described in [RFC3834].

4.7. Interaction with Other Sieve Actions

Vacation does not affect Sieve's implicit keep action.

Vacation can only be executed once per script. A script MUST fail
with an appropriate error if it attempts to execute two or more
vacation actions.

Implementations MUST NOT consider vacation used with discard, keep,
fileinto, or redirect an error. The vacation action is incompatible
with the Sieve reject and refuse actions [REJECT].

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RFC 5230 Sieve: Vacation Extension January 2008

4.8. Examples

Here is a simple use of vacation.

require "vacation";
vacation :days 23 :addresses ["[email protected]",
"[email protected]"]
"I'm away until October 19.
If it's an emergency, call 911, I guess." ;

By mingling vacation with other rules, users can do something more
selective.

require "vacation";
if header :contains "from" "[email protected]" {
redirect "[email protected]";
} else {
vacation "Sorry, I'm away, I'll read your
message when I get around to it.";
}

5. Response Message Generation

This section details the requirements for the generated response
message.

It is worth noting that the input message and arguments may be in
UTF-8, and that implementations MUST deal with UTF-8 input, although
implementations MAY transcode to other character sets as regional
taste dictates. When :mime is used, the reason argument also
contains MIME header information. The headers must conform to MIME
conventions; in particular, 8bit text is not allowed.
Implementations SHOULD reject vacation :mime actions containing 8bit
header material.

5.1. SMTP MAIL FROM Address

The SMTP MAIL FROM address of the message envelope SHOULD be set to
<>. NOTIFY=NEVER SHOULD also be set in the RCPT TO line during the
SMTP transaction if the NOTARY SMTP extension [RFC3461] is available.

5.2. Date

The Date field SHOULD be set to the date and time when the vacation
response was generated. Note that this may not be the same as the
time the message was delivered to the user.

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RFC 5230 Sieve: Vacation Extension January 2008

5.3. Subject

Users can specify the Subject of the reply with the ":subject"
parameter. If the :subject parameter is not supplied, then the
subject is generated as follows: The subject is set to the characters
"Auto: " followed by the original subject. An appropriate fixed
Subject, such as "Automated reply", SHOULD be used in the event that
:subject isn't specified and the original message doesn't contain a
Subject field.

5.4. From

Unless explicitly overridden with a :from parameter, the From field
SHOULD be set to the address of the owner of the Sieve script.

5.5. To

The To field SHOULD be set to the address of the recipient of the
response.

5.6. Auto-Submitted

An Auto-Submitted field with a value of "auto-replied" SHOULD be
included in the message header of any vacation message sent.

5.7. Message Body

The body of the message is taken from the reason string in the
vacation command.

5.8. In-Reply-To and References

Replies MUST have the In-Reply-To field set to the Message-ID of the
original message, and the References field SHOULD be updated with the
Message-ID of the original message.

If the original message lacks a Message-ID, an In-Reply-To need not
be generated, and References need not be changed.

Section 3.6.4 of [RFC2822] provides a complete description of how
References fields should be generated.

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RFC 5230 Sieve: Vacation Extension January 2008

6. Relationship to Recommendations for Automatic Responses to
Electronic Mail

The vacation extension implements a "Personal Responder" in the
terminology defined in [RFC3834]. Care has been taken in this
specification to comply with the recommendations of [RFC3834]
regarding how personal responders should behave.

7. Internationalization Considerations

Internationalization capabilities provided by the base Sieve language
are discussed in [RFC5228]. However, the vacation extension is the
first Sieve extension to be defined that is capable of creating
entirely new messages. This section deals with internationalization
issues raised by the use of the vacation extension.

Vacation messages are normally written using the UTF-8 charset,
allowing text to be written in most of the world's languages.
Additionally, the :mime parameter allows specification of arbitrary
MIME content. In particular, this makes it possible to use
multipart/alternative objects to specify vacation responses in
multiple languages simultaneously.

The Sieve language itself allows a vacation response to be selected
based on the content of the original message. For example, the
Accept-Language or Content-Language header fields [RFC3282] could be
checked and used to select appropriate text:

require "vacation";
if header :contains ["accept-language", "content-language"] "en"
{
vacation "I am away this week.";
} else {
vacation "Estoy ausente esta semana.";
}

Note that this rather simplistic test of the field values fails to
take the structure of the fields into account and hence could be
fooled by some more complex field values. A more elaborate test
could be used to deal with this problem.

The approach of explicitly coding language selection criteria in
scripts is preferred because in many cases language selection issues
are conflated with other selection issues. For example, it may be
appropriate to use informal text in one language for vacation
responses sent to a fellow employee while using more formal text in a
different language in a response sent to a total stranger outside the
company:

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RFC 5230 Sieve: Vacation Extension January 2008

require "vacation";
if address :matches "from" "*@ourdivision.example.com"
{
vacation :subject "Gone fishing"
"Having lots of fun! Back in a day or two!";
} else {
vacation :subject "Je suis parti cette semaine"
"Je lirai votre message quand je retourne.";
}

IMPLEMENTATION NOTE: A graphical Sieve generation interface could in
principle be used to hide the complexity of specifying response
selection criteria from end users. Figuring out the right set of
options to present in a graphical interface is likely a nontrivial
proposition, but this is more because of the need to employ a variety
of criteria to select different sorts of responses to send to
different classes of people than because of the issues involved in
selecting a response in an appropriate language.

8. Security Considerations

It is critical that implementations correctly implement the behavior
and restrictions described throughout this document. Replies MUST
NOT be sent out in response to messages not sent directly to the
user, and replies MUST NOT be sent out more often than the :days
argument states unless the script changes.

If mail is forwarded from a site that uses subaddressing, it may be
impossible to list all recipient addresses with ":addresses".

Security issues associated with mail auto-responders are fully
discussed in the security considerations section of [RFC3834]. This
document is believed not to introduce any additional security
considerations in this general area.

9. IANA Considerations

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

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

Capability name: vacation
Description: adds an action for generating an auto-reply saying
that the original message will not be read or
answered immediately
RFC number: RFC 5230

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RFC 5230 Sieve: Vacation Extension January 2008

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.

10. References

10.1. 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.

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

[RFC3461] Moore, K., "Simple Mail Transfer Protocol (SMTP) Service
Extension for Delivery Status Notifications (DSNs)",
RFC 3461, January 2003.

[RFC3834] Moore, K., "Recommendations for Automatic Responses to
Electronic Mail", RFC 3834, August 2004.

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

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

10.2. Informative References

[REJECT] Stone, A., Elvey, M., and A. Melnikov, "Sieve Email
Filtering: Reject Extension", Work in Progress,
October 2007.

[RFC2142] Crocker, D., "MAILBOX NAMES FOR COMMON SERVICES, ROLES AND
FUNCTIONS", RFC 2142, May 1997.

[RFC2369] Neufeld, G. and J. Baer, "The Use of URLs as Meta-Syntax
for Core Mail List Commands and their Transport through
Message Header Fields", RFC 2369, July 1998.

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RFC 5230 Sieve: Vacation Extension January 2008

[RFC2821] Klensin, J., "Simple Mail Transfer Protocol", RFC 2821,
April 2001.

[RFC2919] Chandhok, R. and G. Wenger, "List-Id: A Structured Field
and Namespace for the Identification of Mailing Lists",
RFC 2919, March 2001.

[RFC3282] Alvestrand, H., "Content Language Headers", RFC 3282,
May 2002.

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RFC 5230 Sieve: Vacation Extension January 2008

Appendix A. Acknowledgements

This extension is obviously inspired by Eric Allman's vacation
program under Unix. The authors owe a great deal to Carnegie Mellon
University, Cyrus Daboo, Lawrence Greenfield, Michael Haardt, Kjetil
Torgrim Homme, Arnt Gulbrandsen, Mark Mallett, Alexey Melnikov,
Jeffrey Hutzelman, Philip Guenther, and many others whose names have
been lost during the inexcusably long gestation period of this
document.

Authors' Addresses

Tim Showalter

EMail: [email protected]

Ned Freed (editor)
Sun Microsystems
3401 Centrelake Drive, Suite 410
Ontario, CA 92761-1205
USA

Phone: +1 909 457 4293
EMail: [email protected]

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RFC 5230 Sieve: Vacation Extension January 2008

Full Copyright Statement

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