| Rfc | 3028 |
|---|
| Title | Sieve: A Mail Filtering Language |
|---|
| Author | T. Showalter |
|---|
| Date | January 2001 |
|---|
| Format: | TXT, HTML |
|---|
| Obsoleted by | RFC5228, RFC5429 |
|---|
| Status: | PROPOSED STANDARD |
|---|
|
Network Working Group T. Showalter
Request for Comments: 3028 Mirapoint, Inc.
Category: Standards Track January 2001
Sieve: A Mail Filtering Language
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) The Internet Society (2001). All Rights Reserved.
Abstract
This document describes a language for filtering e-mail messages at
time of final delivery. It is designed to be implementable on either
a mail client or mail server. It is meant to be extensible, simple,
and independent of access protocol, mail architecture, and operating
system. It is suitable for running on a mail server where users may
not be allowed to execute arbitrary programs, such as on black box
Internet Message Access Protocol (IMAP) servers, as it has no
variables, loops, or ability to shell out to external programs.
Table of Contents
1. Introduction ........................................... 3
1.1. Conventions Used in This Document ..................... 4
1.2. Example mail messages ................................. 4
2. Design ................................................. 5
2.1. Form of the Language .................................. 5
2.2. Whitespace ............................................ 5
2.3. Comments .............................................. 6
2.4. Literal Data .......................................... 6
2.4.1. Numbers ............................................... 6
2.4.2. Strings ............................................... 7
2.4.2.1. String Lists .......................................... 7
2.4.2.2. Headers ............................................... 8
2.4.2.3. Addresses ............................................. 8
2.4.2.4. MIME Parts ............................................ 9
2.5. Tests ................................................. 9
2.5.1. Test Lists ............................................ 9
2.6. Arguments ............................................. 9
2.6.1. Positional Arguments .................................. 9
2.6.2. Tagged Arguments ...................................... 10
2.6.3. Optional Arguments .................................... 10
2.6.4. Types of Arguments .................................... 10
2.7. String Comparison ..................................... 11
2.7.1. Match Type ............................................ 11
2.7.2. Comparisons Across Character Sets ..................... 12
2.7.3. Comparators ........................................... 12
2.7.4. Comparisons Against Addresses ......................... 13
2.8. Blocks ................................................ 14
2.9. Commands .............................................. 14
2.10. Evaluation ............................................ 15
2.10.1. Action Interaction .................................... 15
2.10.2. Implicit Keep ......................................... 15
2.10.3. Message Uniqueness in a Mailbox ....................... 15
2.10.4. Limits on Numbers of Actions .......................... 16
2.10.5. Extensions and Optional Features ...................... 16
2.10.6. Errors ................................................ 17
2.10.7. Limits on Execution ................................... 17
3. Control Commands ....................................... 17
3.1. Control Structure If .................................. 18
3.2. Control Structure Require ............................. 19
3.3. Control Structure Stop ................................ 19
4. Action Commands ........................................ 19
4.1. Action reject ......................................... 20
4.2. Action fileinto ....................................... 20
4.3. Action redirect ....................................... 21
4.4. Action keep ........................................... 21
4.5. Action discard ........................................ 22
5. Test Commands .......................................... 22
5.1. Test address .......................................... 23
5.2. Test allof ............................................ 23
5.3. Test anyof ............................................ 24
5.4. Test envelope ......................................... 24
5.5. Test exists ........................................... 25
5.6. Test false ............................................ 25
5.7. Test header ........................................... 25
5.8. Test not .............................................. 26
5.9. Test size ............................................. 26
5.10. Test true ............................................. 26
6. Extensibility .......................................... 26
6.1. Capability String ..................................... 27
6.2. IANA Considerations ................................... 28
6.2.1. Template for Capability Registrations ................. 28
6.2.2. Initial Capability Registrations ...................... 28
6.3. Capability Transport .................................. 29
7. Transmission ........................................... 29
8. Parsing ................................................ 30
8.1. Lexical Tokens ........................................ 30
8.2. Grammar ............................................... 31
9. Extended Example ....................................... 32
10. Security Considerations ................................ 34
11. Acknowledgments ........................................ 34
12. Author's Address ....................................... 34
13. References ............................................. 34
14. Full Copyright Statement ............................... 36
1. Introduction
This memo documents a language that can be used to create filters for
electronic mail. It is not tied to any particular operating system or
mail architecture. It requires the use of [IMAIL]-compliant
messages, but should otherwise generalize to many systems.
The language is powerful enough to be useful but limited in order to
allow for a safe server-side filtering system. The intention is to
make it impossible for users to do anything more complex (and
dangerous) than write simple mail filters, along with facilitating
the use of GUIs for filter creation and manipulation. The language is
not Turing-complete: it provides no way to write a loop or a function
and variables are not provided.
Scripts written in Sieve are executed during final delivery, when the
message is moved to the user-accessible mailbox. In systems where
the MTA does final delivery, such as traditional Unix mail, it is
reasonable to sort when the MTA deposits mail into the user's
mailbox.
There are a number of reasons to use a filtering system. Mail
traffic for most users has been increasing due to increased usage of
e-mail, the emergence of unsolicited email as a form of advertising,
and increased usage of mailing lists.
Experience at Carnegie Mellon has shown that if a filtering system is
made available to users, many will make use of it in order to file
messages from specific users or mailing lists. However, many others
did not make use of the Andrew system's FLAMES filtering language
[FLAMES] due to difficulty in setting it up.
Because of the expectation that users will make use of filtering if
it is offered and easy to use, this language has been made simple
enough to allow many users to make use of it, but rich enough that it
can be used productively. However, it is expected that GUI-based
editors will be the preferred way of editing filters for a large
number of users.
1.1. Conventions Used in This Document
In the sections of this document that discuss the requirements of
various keywords and operators, the following conventions have been
adopted.
The key words "MUST", "MUST NOT", "SHOULD", "SHOULD NOT", and
"MAY" in this document are to be interpreted as defined in
[KEYWORDS].
Each section on a command (test, action, or control structure) has a
line labeled "Syntax:". This line describes the syntax of the
command, including its name and its arguments. Required arguments
are listed inside angle brackets ("<" and ">"). Optional arguments
are listed inside square brackets ("[" and "]"). Each argument is
followed by its type, so "<key: string>" represents an argument
called "key" that is a string. Literal strings are represented with
double-quoted strings. Alternatives are separated with slashes, and
parenthesis are used for grouping, similar to [ABNF].
In the "Syntax" line, there are three special pieces of syntax that
are frequently repeated, MATCH-TYPE, COMPARATOR, and ADDRESS-PART.
These are discussed in sections 2.7.1, 2.7.3, and 2.7.4,
respectively.
The formal grammar for these commands in section 10 and is the
authoritative reference on how to construct commands, but the formal
grammar does not specify the order, semantics, number or types of
arguments to commands, nor the legal command names. The intent is to
allow for extension without changing the grammar.
1.2. Example mail messages
The following mail messages will be used throughout this document in
examples.
Message A
-----------------------------------------------------------
Date: Tue, 1 Apr 1997 09:06:31 -0800 (PST)
From: coyote@desert.example.org
To: roadrunner@acme.example.com
Subject: I have a present for you
Look, I'm sorry about the whole anvil thing, and I really
didn't mean to try and drop it on you from the top of the
cliff. I want to try to make it up to you. I've got some
great birdseed over here at my place--top of the line
stuff--and if you come by, I'll have it all wrapped up
for you. I'm really sorry for all the problems I've caused
for you over the years, but I know we can work this out.
--
Wile E. Coyote "Super Genius" coyote@desert.example.org
-----------------------------------------------------------
Message B
-----------------------------------------------------------
From: youcouldberich!@reply-by-postal-mail.invalid
Sender: b1ff@de.res.example.com
To: rube@landru.example.edu
Date: Mon, 31 Mar 1997 18:26:10 -0800
Subject: $$$ YOU, TOO, CAN BE A MILLIONAIRE! $$$
YOU MAY HAVE ALREADY WON TEN MILLION DOLLARS, BUT I DOUBT
IT! SO JUST POST THIS TO SIX HUNDRED NEWSGROUPS! IT WILL
GUARANTEE THAT YOU GET AT LEAST FIVE RESPONSES WITH MONEY!
MONEY! MONEY! COLD HARD CASH! YOU WILL RECEIVE OVER
$20,000 IN LESS THAN TWO MONTHS! AND IT'S LEGAL!!!!!!!!!
!!!!!!!!!!!!!!!!!!111111111!!!!!!!11111111111!!1 JUST
SEND $5 IN SMALL, UNMARKED BILLS TO THE ADDRESSES BELOW!
-----------------------------------------------------------
2. Design
2.1. Form of the Language
The language consists of a set of commands. Each command consists of
a set of tokens delimited by whitespace. The command identifier is
the first token and it is followed by zero or more argument tokens.
Arguments may be literal data, tags, blocks of commands, or test
commands.
The language is represented in UTF-8, as specified in [UTF-8].
Tokens in the ASCII range are considered case-insensitive.
2.2. Whitespace
Whitespace is used to separate tokens. Whitespace is made up of
tabs, newlines (CRLF, never just CR or LF), and the space character.
The amount of whitespace used is not significant.
2.3. Comments
Two types of comments are offered. Comments are semantically
equivalent to whitespace and can be used anyplace that whitespace is
(with one exception in multi-line strings, as described in the
grammar).
Hash comments begin with a "#" character that is not contained within
a string and continue until the next CRLF.
Example: if size :over 100K { # this is a comment
discard;
}
Bracketed comments begin with the token "/*" and end with "*/" outside
of a string. Bracketed comments may span multiple lines. Bracketed
comments do not nest.
Example: if size :over 100K { /* this is a comment
this is still a comment */ discard /* this is a comment
*/ ;
}
2.4. Literal Data
Literal data means data that is not executed, merely evaluated "as
is", to be used as arguments to commands. Literal data is limited to
numbers and strings.
2.4.1. Numbers
Numbers are given as ordinary decimal numbers. However, those
numbers that have a tendency to be fairly large, such as message
sizes, MAY have a "K", "M", or "G" appended to indicate a multiple of
a power of two. To be comparable with the power-of-two-based
versions of SI units that computers frequently use, K specifies
kibi-, or 1,024 (2^10) times the value of the number; M specifies
mebi-, or 1,048,576 (2^20) times the value of the number; and G
specifies tebi-, or 1,073,741,824 (2^30) times the value of the
number [BINARY-SI].
Implementations MUST provide 31 bits of magnitude in numbers, but MAY
provide more.
Only positive integers are permitted by this specification.
2.4.2. Strings
Scripts involve large numbers of strings as they are used for pattern
matching, addresses, textual bodies, etc. Typically, short quoted
strings suffice for most uses, but a more convenient form is provided
for longer strings such as bodies of messages.
A quoted string starts and ends with a single double quote (the <">
character, ASCII 34). A backslash ("\", ASCII 92) inside of a quoted
string is followed by either another backslash or a double quote.
This two-character sequence represents a single backslash or double-
quote within the string, respectively.
No other characters should be escaped with a single backslash.
An undefined escape sequence (such as "\a" in a context where "a" has
no special meaning) is interpreted as if there were no backslash (in
this case, "\a" is just "a").
Non-printing characters such as tabs, CR and LF, and control
characters are permitted in quoted strings. Quoted strings MAY span
multiple lines. NUL (ASCII 0) is not allowed in strings.
For entering larger amounts of text, such as an email message, a
multi-line form is allowed. It starts with the keyword "text:",
followed by a CRLF, and ends with the sequence of a CRLF, a single
period, and another CRLF. In order to allow the message to contain
lines with a single-dot, lines are dot-stuffed. That is, when
composing a message body, an extra `.' is added before each line
which begins with a `.'. When the server interprets the script,
these extra dots are removed. Note that a line that begins with a
dot followed by a non-dot character is not interpreted dot-stuffed;
that is, ".foo" is interpreted as ".foo". However, because this is
potentially ambiguous, scripts SHOULD be properly dot-stuffed so such
lines do not appear.
Note that a hashed comment or whitespace may occur in between the
"text:" and the CRLF, but not within the string itself. Bracketed
comments are not allowed here.
2.4.2.1. String Lists
When matching patterns, it is frequently convenient to match against
groups of strings instead of single strings. For this reason, a list
of strings is allowed in many tests, implying that if the test is
true using any one of the strings, then the test is true.
Implementations are encouraged to use short-circuit evaluation in
these cases.
For instance, the test `header :contains ["To", "Cc"]
["me@example.com", "me00@landru.example.edu"]' is true if either the
To header or Cc header of the input message contains either of the
e-mail addresses "me@example.com" or "me00@landru.example.edu".
Conversely, in any case where a list of strings is appropriate, a
single string is allowed without being a member of a list: it is
equivalent to a list with a single member. This means that the test
`exists "To"' is equivalent to the test `exists ["To"]'.
2.4.2.2. Headers
Headers are a subset of strings. In the Internet Message
Specification [IMAIL] [RFC1123], each header line is allowed to have
whitespace nearly anywhere in the line, including after the field
name and before the subsequent colon. Extra spaces between the
header name and the ":" in a header field are ignored.
A header name never contains a colon. The "From" header refers to a
line beginning "From:" (or "From :", etc.). No header will match
the string "From:" due to the trailing colon.
Folding of long header lines (as described in [IMAIL] 3.4.8) is
removed prior to interpretation of the data. The folding syntax (the
CRLF that ends a line plus any leading whitespace at the beginning of
the next line that indicates folding) are interpreted as if they were
a single space.
2.4.2.3. Addresses
A number of commands call for email addresses, which are also a
subset of strings. When these addresses are used in outbound
contexts, addresses must be compliant with [IMAIL], but are further
constrained. Using the symbols defined in [IMAIL], section 6.1, the
syntax of an address is:
sieve-address = addr-spec ; simple address
/ phrase "<" addr-spec ">" ; name & addr-spec
That is, routes and group syntax are not permitted. If multiple
addresses are required, use a string list. Named groups are not used
here.
Implementations MUST ensure that the addresses are syntactically
valid, but need not ensure that they actually identify an email
recipient.
2.4.2.4. MIME Parts
In a few places, [MIME] body parts are represented as strings. These
parts include MIME headers and the body. This provides a way of
embedding typed data within a Sieve script so that, among other
things, character sets other than UTF-8 can be used for output
messages.
2.5. Tests
Tests are given as arguments to commands in order to control their
actions. In this document, tests are given to if/elsif/else to
decide which block of code is run.
Tests MUST NOT have side effects. That is, a test cannot affect the
state of the filter or message. No tests in this specification have
side effects, and side effects are forbidden in extension tests as
well.
The rationale for this is that tests with side effects impair
readability and maintainability and are difficult to represent in a
graphic interface for generating scripts. Side effects are confined
to actions where they are clearer.
2.5.1. Test Lists
Some tests ("allof" and "anyof", which implement logical "and" and
logical "or", respectively) may require more than a single test as an
argument. The test-list syntax element provides a way of grouping
tests.
Example: if anyof (not exists ["From", "Date"],
header :contains "from" "fool@example.edu") {
discard;
}
2.6. Arguments
In order to specify what to do, most commands take arguments. There
are three types of arguments: positional, tagged, and optional.
2.6.1. Positional Arguments
Positional arguments are given to a command which discerns their
meaning based on their order. When a command takes positional
arguments, all positional arguments must be supplied and must be in
the order prescribed.
2.6.2. Tagged Arguments
This document provides for tagged arguments in the style of
CommonLISP. These are also similar to flags given to commands in
most command-line systems.
A tagged argument is an argument for a command that begins with ":"
followed by a tag naming the argument, such as ":contains". This
argument means that zero or more of the next tokens have some
particular meaning depending on the argument. These next tokens may
be numbers or strings but they are never blocks.
Tagged arguments are similar to positional arguments, except that
instead of the meaning being derived from the command, it is derived
from the tag.
Tagged arguments must appear before positional arguments, but they
may appear in any order with other tagged arguments. For simplicity
of the specification, this is not expressed in the syntax definitions
with commands, but they still may be reordered arbitrarily provided
they appear before positional arguments. Tagged arguments may be
mixed with optional arguments.
To simplify this specification, tagged arguments SHOULD NOT take
tagged arguments as arguments.
2.6.3. Optional Arguments
Optional arguments are exactly like tagged arguments except that they
may be left out, in which case a default value is implied. Because
optional arguments tend to result in shorter scripts, they have been
used far more than tagged arguments.
One particularly noteworthy case is the ":comparator" argument, which
allows the user to specify which [ACAP] comparator will be used to
compare two strings, since different languages may impose different
orderings on UTF-8 [UTF-8] characters.
2.6.4. Types of Arguments
Abstractly, arguments may be literal data, tests, or blocks of
commands. In this way, an "if" control structure is merely a command
that happens to take a test and a block as arguments and may execute
the block of code.
However, this abstraction is ambiguous from a parsing standpoint.
The grammar in section 9.2 presents a parsable version of this:
Arguments are string-lists, numbers, and tags, which may be followed
by a test or a test-list, which may be followed by a block of
commands. No more than one test or test list, nor more than one
block of commands, may be used, and commands that end with blocks of
commands do not end with semicolons.
2.7. String Comparison
When matching one string against another, there are a number of ways
of performing the match operation. These are accomplished with three
types of matches: an exact match, a substring match, and a wildcard
glob-style match. These are described below.
In order to provide for matches between character sets and case
insensitivity, Sieve borrows ACAP's comparator registry.
However, when a string represents the name of a header, the
comparator is never user-specified. Header comparisons are always
done with the "i;ascii-casemap" operator, i.e., case-insensitive
comparisons, because this is the way things are defined in the
message specification [IMAIL].
2.7.1. Match Type
There are three match types describing the matching used in this
specification: ":is", ":contains", and ":matches". Match type
arguments are supplied to those commands which allow them to specify
what kind of match is to be performed.
These are used as tagged arguments to tests that perform string
comparison.
The ":contains" match type describes a substring match. If the value
argument contains the key argument as a substring, the match is true.
For instance, the string "frobnitzm" contains "frob" and "nit", but
not "fbm". The null key ("") is contained in all values.
The ":is" match type describes an absolute match; if the contents of
the first string are absolutely the same as the contents of the
second string, they match. Only the string "frobnitzm" is the string
"frobnitzm". The null key ":is" and only ":is" the null value.
The ":matches" version specifies a wildcard match using the
characters "*" and "?". "*" matches zero or more characters, and "?"
matches a single character. "?" and "*" may be escaped as "\\?" and
"\\*" in strings to match against themselves. The first backslash
escapes the second backslash; together, they escape the "*". This is
awkward, but it is commonplace in several programming languages that
use globs and regular expressions.
In order to specify what type of match is supposed to happen,
commands that support matching take optional tagged arguments
":matches", ":is", and ":contains". Commands default to using ":is"
matching if no match type argument is supplied. Note that these
modifiers may interact with comparators; in particular, some
comparators are not suitable for matching with ":contains" or
":matches". It is an error to use a comparator with ":contains" or
":matches" that is not compatible with it.
It is an error to give more than one of these arguments to a given
command.
For convenience, the "MATCH-TYPE" syntax element is defined here as
follows:
Syntax: ":is" / ":contains" / ":matches"
2.7.2. Comparisons Across Character Sets
All Sieve scripts are represented in UTF-8, but messages may involve
a number of character sets. In order for comparisons to work across
character sets, implementations SHOULD implement the following
behavior:
Implementations decode header charsets to UTF-8. Two strings are
considered equal if their UTF-8 representations are identical.
Implementations should decode charsets represented in the forms
specified by [MIME] for both message headers and bodies.
Implementations must be capable of decoding US-ASCII, ISO-8859-1,
the ASCII subset of ISO-8859-* character sets, and UTF-8.
If implementations fail to support the above behavior, they MUST
conform to the following:
No two strings can be considered equal if one contains octets
greater than 127.
2.7.3. Comparators
In order to allow for language-independent, case-independent matches,
the match type may be coupled with a comparator name. Comparators
are described for [ACAP]; a registry is defined for ACAP, and this
specification uses that registry.
ACAP defines multiple comparator types. Only equality types are used
in this specification.
All implementations MUST support the "i;octet" comparator (simply
compares octets) and the "i;ascii-casemap" comparator (which treats
uppercase and lowercase characters in the ASCII subset of UTF-8 as
the same). If left unspecified, the default is "i;ascii-casemap".
Some comparators may not be usable with substring matches; that is,
they may only work with ":is". It is an error to try and use a
comparator with ":matches" or ":contains" that is not compatible with
it.
A comparator is specified by the ":comparator" option with commands
that support matching. This option is followed by a string providing
the name of the comparator to be used. For convenience, the syntax
of a comparator is abbreviated to "COMPARATOR", and (repeated in
several tests) is as follows:
Syntax: ":comparator" <comparator-name: string>
So in this example,
Example: if header :contains :comparator "i;octet" "Subject"
"MAKE MONEY FAST" {
discard;
}
would discard any message with subjects like "You can MAKE MONEY
FAST", but not "You can Make Money Fast", since the comparator used
is case-sensitive.
Comparators other than i;octet and i;ascii-casemap must be declared
with require, as they are extensions. If a comparator declared with
require is not known, it is an error, and execution fails. If the
comparator is not declared with require, it is also an error, even if
the comparator is supported. (See 2.10.5.)
Both ":matches" and ":contains" match types are compatible with the
"i;octet" and "i;ascii-casemap" comparators and may be used with
them.
It is an error to give more than one of these arguments to a given
command.
2.7.4. Comparisons Against Addresses
Addresses are one of the most frequent things represented as strings.
These are structured, and being able to compare against the local-
part or the domain of an address is useful, so some tests that act
exclusively on addresses take an additional optional argument that
specifies what the test acts on.
These optional arguments are ":localpart", ":domain", and ":all",
which act on the local-part (left-side), the domain part (right-
side), and the whole address.
The kind of comparison done, such as whether or not the test done is
case-insensitive, is specified as a comparator argument to the test.
If an optional address-part is omitted, the default is ":all".
It is an error to give more than one of these arguments to a given
command.
For convenience, the "ADDRESS-PART" syntax element is defined here as
follows:
Syntax: ":localpart" / ":domain" / ":all"
2.8. Blocks
Blocks are sets of commands enclosed within curly braces. Blocks are
supplied to commands so that the commands can implement control
commands.
A control structure is a command that happens to take a test and a
block as one of its arguments; depending on the result of the test
supplied as another argument, it runs the code in the block some
number of times.
With the commands supplied in this memo, there are no loops. The
control structures supplied--if, elsif, and else--run a block either
once or not at all. So there are two arguments, the test and the
block.
2.9. Commands
Sieve scripts are sequences of commands. Commands can take any of
the tokens above as arguments, and arguments may be either tagged or
positional arguments. Not all commands take all arguments.
There are three kinds of commands: test commands, action commands,
and control commands.
The simplest is an action command. An action command is an
identifier followed by zero or more arguments, terminated by a
semicolon. Action commands do not take tests or blocks as arguments.
A control command is similar, but it takes a test as an argument, and
ends with a block instead of a semicolon.
A test command is used as part of a control command. It is used to
specify whether or not the block of code given to the control command
is executed.
2.10. Evaluation
2.10.1. Action Interaction
Some actions cannot be used with other actions because the result
would be absurd. These restrictions are noted throughout this memo.
Extension actions MUST state how they interact with actions defined
in this specification.
2.10.2. Implicit Keep
Previous experience with filtering systems suggests that cases tend
to be missed in scripts. To prevent errors, Sieve has an "implicit
keep".
An implicit keep is a keep action (see 4.4) performed in absence of
any action that cancels the implicit keep.
An implicit keep is performed if a message is not written to a
mailbox, redirected to a new address, or explicitly thrown out. That
is, if a fileinto, a keep, a redirect, or a discard is performed, an
implicit keep is not.
Some actions may be defined to not cancel the implicit keep. These
actions may not directly affect the delivery of a message, and are
used for their side effects. None of the actions specified in this
document meet that criteria, but extension actions will.
For instance, with any of the short messages offered above, the
following script produces no actions.
Example: if size :over 500K { discard; }
As a result, the implicit keep is taken.
2.10.3. Message Uniqueness in a Mailbox
Implementations SHOULD NOT deliver a message to the same folder more
than once, even if a script explicitly asks for a message to be
written to a mailbox twice.
The test for equality of two messages is implementation-defined.
If a script asks for a message to be written to a mailbox twice, it
MUST NOT be treated as an error.
2.10.4. Limits on Numbers of Actions
Site policy MAY limit numbers of actions taken and MAY impose
restrictions on which actions can be used together. In the event
that a script hits a policy limit on the number of actions taken for
a particular message, an error occurs.
Implementations MUST prohibit more than one reject.
Implementations MUST allow at least one keep or one fileinto. If
fileinto is not implemented, implementations MUST allow at least one
keep.
Implementations SHOULD prohibit reject when used with other actions.
2.10.5. Extensions and Optional Features
Because of the differing capabilities of many mail systems, several
features of this specification are optional. Before any of these
extensions can be executed, they must be declared with the "require"
action.
If an extension is not enabled with "require", implementations MUST
treat it as if they did not support it at all.
If a script does not understand an extension declared with require,
the script must not be used at all. Implementations MUST NOT execute
scripts which require unknown capability names.
Note: The reason for this restriction is that prior experiences with
languages such as LISP and Tcl suggest that this is a workable
way of noting that a given script uses an extension.
Experience with PostScript suggests that mechanisms that allow
a script to work around missing extensions are not used in
practice.
Extensions which define actions MUST state how they interact with
actions discussed in the base specification.
2.10.6. Errors
In any programming language, there are compile-time and run-time
errors.
Compile-time errors are ones in syntax that are detectable if a
syntax check is done.
Run-time errors are not detectable until the script is run. This
includes transient failures like disk full conditions, but also
includes issues like invalid combinations of actions.
When an error occurs in a Sieve script, all processing stops.
Implementations MAY choose to do a full parse, then evaluate the
script, then do all actions. Implementations might even go so far as
to ensure that execution is atomic (either all actions are executed
or none are executed).
Other implementations may choose to parse and run at the same time.
Such implementations are simpler, but have issues with partial
failure (some actions happen, others don't).
Implementations might even go so far as to ensure that scripts can
never execute an invalid set of actions (e.g., reject + fileinto)
before execution, although this could involve solving the Halting
Problem.
This specification allows any of these approaches. Solving the
Halting Problem is considered extra credit.
When an error happens, implementations MUST notify the user that an
error occurred, which actions (if any) were taken, and do an implicit
keep.
2.10.7. Limits on Execution
Implementations may limit certain constructs. However, this
specification places a lower bound on some of these limits.
Implementations MUST support fifteen levels of nested blocks.
Implementations MUST support fifteen levels of nested test lists.
3. Control Commands
Control structures are needed to allow for multiple and conditional
actions.
3.1. Control Structure If
There are three pieces to if: "if", "elsif", and "else". Each is
actually a separate command in terms of the grammar. However, an
elsif MUST only follow an if, and an else MUST follow only either an
if or an elsif. An error occurs if these conditions are not met.
Syntax: if <test1: test> <block1: block>
Syntax: elsif <test2: test> <block2: block>
Syntax: else <block>
The semantics are similar to those of any of the many other
programming languages these control commands appear in. When the
interpreter sees an "if", it evaluates the test associated with it.
If the test is true, it executes the block associated with it.
If the test of the "if" is false, it evaluates the test of the first
"elsif" (if any). If the test of "elsif" is true, it runs the
elsif's block. An elsif may be followed by an elsif, in which case,
the interpreter repeats this process until it runs out of elsifs.
When the interpreter runs out of elsifs, there may be an "else" case.
If there is, and none of the if or elsif tests were true, the
interpreter runs the else case.
This provides a way of performing exactly one of the blocks in the
chain.
In the following example, both Message A and B are dropped.
Example: require "fileinto";
if header :contains "from" "coyote" {
discard;
} elsif header :contains ["subject"] ["$$$"] {
discard;
} else {
fileinto "INBOX";
}
When the script below is run over message A, it redirects the message
to acm@example.edu; message B, to postmaster@example.edu; any other
message is redirected to field@example.edu.
Example: if header :contains ["From"] ["coyote"] {
redirect "acm@example.edu";
} elsif header :contains "Subject" "$$$" {
redirect "postmaster@example.edu";
} else {
redirect "field@example.edu";
}
Note that this definition prohibits the "... else if ..." sequence
used by C. This is intentional, because this construct produces a
shift-reduce conflict.
3.2. Control Structure Require
Syntax: require <capabilities: string-list>
The require action notes that a script makes use of a certain
extension. Such a declaration is required to use the extension, as
discussed in section 2.10.5. Multiple capabilities can be declared
with a single require.
The require command, if present, MUST be used before anything other
than a require can be used. An error occurs if a require appears
after a command other than require.
Example: require ["fileinto", "reject"];
Example: require "fileinto";
require "vacation";
3.3. Control Structure Stop
Syntax: stop
The "stop" action ends all processing. If no actions have been
executed, then the keep action is taken.
4. Action Commands
This document supplies five actions that may be taken on a message:
keep, fileinto, redirect, reject, and discard.
Implementations MUST support the "keep", "discard", and "redirect"
actions.
Implementations SHOULD support "reject" and "fileinto".
Implementations MAY limit the number of certain actions taken (see
section 2.10.4).
4.1. Action reject
Syntax: reject <reason: string>
The optional "reject" action refuses delivery of a message by sending
back an [MDN] to the sender. It resends the message to the sender,
wrapping it in a "reject" form, noting that it was rejected by the
recipient. In the following script, message A is rejected and
returned to the sender.
Example: if header :contains "from" "coyote@desert.example.org" {
reject "I am not taking mail from you, and I don't want
your birdseed, either!";
}
A reject message MUST take the form of a failure MDN as specified by
[MDN]. The human-readable portion of the message, the first
component of the MDN, contains the human readable message describing
the error, and it SHOULD contain additional text alerting the
original sender that mail was refused by a filter. This part of the
MDN might appear as follows:
------------------------------------------------------------
Message was refused by recipient's mail filtering program. Reason
given was as follows:
I am not taking mail from you, and I don't want your birdseed,
either!
------------------------------------------------------------
The MDN action-value field as defined in the MDN specification MUST
be "deleted" and MUST have the MDN-sent-automatically and automatic-
action modes set.
Because some implementations can not or will not implement the reject
command, it is optional. The capability string to be used with the
require command is "reject".
4.2. Action fileinto
Syntax: fileinto <folder: string>
The "fileinto" action delivers the message into the specified folder.
Implementations SHOULD support fileinto, but in some environments
this may be impossible.
The capability string for use with the require command is "fileinto".
In the following script, message A is filed into folder
"INBOX.harassment".
Example: require "fileinto";
if header :contains ["from"] "coyote" {
fileinto "INBOX.harassment";
}
4.3. Action redirect
Syntax: redirect <address: string>
The "redirect" action is used to send the message to another user at
a supplied address, as a mail forwarding feature does. The
"redirect" action makes no changes to the message body or existing
headers, but it may add new headers. The "redirect" modifies the
envelope recipient.
The redirect command performs an MTA-style "forward"--that is, what
you get from a .forward file using sendmail under UNIX. The address
on the SMTP envelope is replaced with the one on the redirect command
and the message is sent back out. (This is not an MUA-style forward,
which creates a new message with a different sender and message ID,
wrapping the old message in a new one.)
A simple script can be used for redirecting all mail:
Example: redirect "bart@example.edu";
Implementations SHOULD take measures to implement loop control,
possibly including adding headers to the message or counting received
headers. If an implementation detects a loop, it causes an error.
4.4. Action keep
Syntax: keep
The "keep" action is whatever action is taken in lieu of all other
actions, if no filtering happens at all; generally, this simply means
to file the message into the user's main mailbox. This command
provides a way to execute this action without needing to know the
name of the user's main mailbox, providing a way to call it without
needing to understand the user's setup, or the underlying mail
system.
For instance, in an implementation where the IMAP server is running
scripts on behalf of the user at time of delivery, a keep command is
equivalent to a fileinto "INBOX".
Example: if size :under 1M { keep; } else { discard; }
Note that the above script is identical to the one below.
Example: if not size :under 1M { discard; }
4.5. Action discard
Syntax: discard
Discard is used to silently throw away the message. It does so by
simply canceling the implicit keep. If discard is used with other
actions, the other actions still happen. Discard is compatible with
all other actions. (For instance fileinto+discard is equivalent to
fileinto.)
Discard MUST be silent; that is, it MUST NOT return a non-delivery
notification of any kind ([DSN], [MDN], or otherwise).
In the following script, any mail from "idiot@example.edu" is thrown
out.
Example: if header :contains ["from"] ["idiot@example.edu"] {
discard;
}
While an important part of this language, "discard" has the potential
to create serious problems for users: Students who leave themselves
logged in to an unattended machine in a public computer lab may find
their script changed to just "discard". In order to protect users in
this situation (along with similar situations), implementations MAY
keep messages destroyed by a script for an indefinite period, and MAY
disallow scripts that throw out all mail.
5. Test Commands
Tests are used in conditionals to decide which part(s) of the
conditional to execute.
Implementations MUST support these tests: "address", "allof",
"anyof", "exists", "false", "header", "not", "size", and "true".
Implementations SHOULD support the "envelope" test.
5.1. Test address
Syntax: address [ADDRESS-PART] [COMPARATOR] [MATCH-TYPE]
<header-list: string-list> <key-list: string-list>
The address test matches Internet addresses in structured headers
that contain addresses. It returns true if any header contains any
key in the specified part of the address, as modified by the
comparator and the match keyword.
Like envelope and header, this test returns true if any combination
of the header-list and key-list arguments match.
Internet email addresses [IMAIL] have the somewhat awkward
characteristic that the local-part to the left of the at-sign is
considered case sensitive, and the domain-part to the right of the
at-sign is case insensitive. The "address" command does not deal
with this itself, but provides the ADDRESS-PART argument for allowing
users to deal with it.
The address primitive never acts on the phrase part of an email
address, nor on comments within that address. It also never acts on
group names, although it does act on the addresses within the group
construct.
Implementations MUST restrict the address test to headers that
contain addresses, but MUST include at least From, To, Cc, Bcc,
Sender, Resent-From, Resent-To, and SHOULD include any other header
that utilizes an "address-list" structured header body.
Example: if address :is :all "from" "tim@example.com" {
discard;
5.2. Test allof
Syntax: allof <tests: test-list>
The allof test performs a logical AND on the tests supplied to it.
Example: allof (false, false) => false
allof (false, true) => false
allof (true, true) => true
The allof test takes as its argument a test-list.
5.3. Test anyof
Syntax: anyof <tests: test-list>
The anyof test performs a logical OR on the tests supplied to it.
Example: anyof (false, false) => false
anyof (false, true) => true
anyof (true, true) => true
5.4. Test envelope
Syntax: envelope [COMPARATOR] [ADDRESS-PART] [MATCH-TYPE]
<envelope-part: string-list> <key-list: string-list>
The "envelope" test is true if the specified part of the SMTP (or
equivalent) envelope matches the specified key.
If one of the envelope-part strings is (case insensitive) "from",
then matching occurs against the FROM address used in the SMTP MAIL
command.
If one of the envelope-part strings is (case insensitive) "to", then
matching occurs against the TO address used in the SMTP RCPT command
that resulted in this message getting delivered to this user. Note
that only the most recent TO is available, and only the one relevant
to this user.
The envelope-part is a string list and may contain more than one
parameter, in which case all of the strings specified in the key-list
are matched against all parts given in the envelope-part list.
Like address and header, this test returns true if any combination of
the envelope-part and key-list arguments is true.
All tests against envelopes MUST drop source routes.
If the SMTP transaction involved several RCPT commands, only the data
from the RCPT command that caused delivery to this user is available
in the "to" part of the envelope.
If a protocol other than SMTP is used for message transport,
implementations are expected to adapt this command appropriately.
The envelope command is optional. Implementations SHOULD support it,
but the necessary information may not be available in all cases.
Example: require "envelope";
if envelope :all :is "from" "tim@example.com" {
discard;
}
5.5. Test exists
Syntax: exists <header-names: string-list>
The "exists" test is true if the headers listed in the header-names
argument exist within the message. All of the headers must exist or
the test is false.
The following example throws out mail that doesn't have a From header
and a Date header.
Example: if not exists ["From","Date"] {
discard;
}
5.6. Test false
Syntax: false
The "false" test always evaluates to false.
5.7. Test header
Syntax: header [COMPARATOR] [MATCH-TYPE]
<header-names: string-list> <key-list: string-list>
The "header" test evaluates to true if any header name matches any
key. The type of match is specified by the optional match argument,
which defaults to ":is" if not specified, as specified in section
2.6.
Like address and envelope, this test returns true if any combination
of the string-list and key-list arguments match.
If a header listed in the header-names argument exists, it contains
the null key (""). However, if the named header is not present, it
does not contain the null key. So if a message contained the header
X-Caffeine: C8H10N4O2
these tests on that header evaluate as follows:
header :is ["X-Caffeine"] [""] => false
header :contains ["X-Caffeine"] [""] => true
5.8. Test not
Syntax: not <test>
The "not" test takes some other test as an argument, and yields the
opposite result. "not false" evaluates to "true" and "not true"
evaluates to "false".
5.9. Test size
Syntax: size <":over" / ":under"> <limit: number>
The "size" test deals with the size of a message. It takes either a
tagged argument of ":over" or ":under", followed by a number
representing the size of the message.
If the argument is ":over", and the size of the message is greater
than the number provided, the test is true; otherwise, it is false.
If the argument is ":under", and the size of the message is less than
the number provided, the test is true; otherwise, it is false.
Exactly one of ":over" or ":under" must be specified, and anything
else is an error.
The size of a message is defined to be the number of octets from the
initial header until the last character in the message body.
Note that for a message that is exactly 4,000 octets, the message is
neither ":over" 4000 octets or ":under" 4000 octets.
5.10. Test true
Syntax: true
The "true" test always evaluates to true.
6. Extensibility
New control structures, actions, and tests can be added to the
language. Sites must make these features known to their users; this
document does not define a way to discover the list of extensions
supported by the server.
Any extensions to this language MUST define a capability string that
uniquely identifies that extension. If a new version of an extension
changes the functionality of a previously defined extension, it MUST
use a different name.
In a situation where there is a submission protocol and an extension
advertisement mechanism aware of the details of this language,
scripts submitted can be checked against the mail server to prevent
use of an extension that the server does not support.
Extensions MUST state how they interact with constraints defined in
section 2.10, e.g., whether they cancel the implicit keep, and which
actions they are compatible and incompatible with.
6.1. Capability String
Capability strings are typically short strings describing what
capabilities are supported by the server.
Capability strings beginning with "vnd." represent vendor-defined
extensions. Such extensions are not defined by Internet standards or
RFCs, but are still registered with IANA in order to prevent
conflicts. Extensions starting with "vnd." SHOULD be followed by the
name of the vendor and product, such as "vnd.acme.rocket-sled".
The following capability strings are defined by this document:
envelope The string "envelope" indicates that the implementation
supports the "envelope" command.
fileinto The string "fileinto" indicates that the implementation
supports the "fileinto" command.
reject The string "reject" indicates that the implementation
supports the "reject" command.
comparator- The string "comparator-elbonia" is provided if the
implementation supports the "elbonia" comparator.
Therefore, all implementations have at least the
"comparator-i;octet" and "comparator-i;ascii-casemap"
capabilities. However, these comparators may be used
without being declared with require.
6.2. IANA Considerations
In order to provide a standard set of extensions, a registry is
provided by IANA. Capability names may be registered on a first-
come, first-served basis. Extensions designed for interoperable use
SHOULD be defined as standards track or IESG approved experimental
RFCs.
6.2.1. Template for Capability Registrations
The following template is to be used for registering new Sieve
extensions with IANA.
To: iana@iana.org
Subject: Registration of new Sieve extension
Capability name:
Capability keyword:
Capability arguments:
Standards Track/IESG-approved experimental RFC number:
Person and email address to contact for further information:
6.2.2. Initial Capability Registrations
The following are to be added to the IANA registry for Sieve
extensions as the initial contents of the capability registry.
Capability name: fileinto
Capability keyword: fileinto
Capability arguments: fileinto <folder: string>
Standards Track/IESG-approved experimental RFC number:
RFC 3028 (Sieve base spec)
Person and email address to contact for further information:
Tim Showalter
tjs@mirapoint.com
Capability name: reject
Capability keyword: reject
Capability arguments: reject <reason: string>
Standards Track/IESG-approved experimental RFC number:
RFC 3028 (Sieve base spec)
Person and email address to contact for further information:
Tim Showalter
tjs@mirapoint.com
Capability name: envelope
Capability keyword: envelope
Capability arguments:
envelope [COMPARATOR] [ADDRESS-PART] [MATCH-TYPE]
<envelope-part: string-list> <key-list: string-list>
Standards Track/IESG-approved experimental RFC number:
RFC 3028 (Sieve base spec)
Person and email address to contact for further information:
Tim Showalter
tjs@mirapoint.com
Capability name: comparator-*
Capability keyword:
comparator-* (anything starting with "comparator-")
Capability arguments: (none)
Standards Track/IESG-approved experimental RFC number:
RFC 3028, Sieve, by reference of
RFC 2244, Application Configuration Access Protocol
Person and email address to contact for further information:
Tim Showalter
tjs@mirapoint.com
6.3. Capability Transport
As the range of mail systems that this document is intended to apply
to is quite varied, a method of advertising which capabilities an
implementation supports is difficult due to the wide range of
possible implementations. Such a mechanism, however, should have
property that the implementation can advertise the complete set of
extensions that it supports.
7. Transmission
The MIME type for a Sieve script is "application/sieve".
The registration of this type for RFC 2048 requirements is as
follows:
Subject: Registration of MIME media type application/sieve
MIME media type name: application
MIME subtype name: sieve
Required parameters: none
Optional parameters: none
Encoding considerations: Most sieve scripts will be textual,
written in UTF-8. When non-7bit characters are used,
quoted-printable is appropriate for transport systems
that require 7bit encoding.
Security considerations: Discussed in section 10 of RFC 3028.
Interoperability considerations: Discussed in section 2.10.5
of RFC 3028.
Published specification: RFC 3028.
Applications which use this media type: sieve-enabled mail servers
Additional information:
Magic number(s):
File extension(s): .siv
Macintosh File Type Code(s):
Person & email address to contact for further information:
See the discussion list at ietf-mta-filters@imc.org.
Intended usage:
COMMON
Author/Change controller:
See Author information in RFC 3028.
8. Parsing
The Sieve grammar is separated into tokens and a separate grammar as
most programming languages are.
8.1. Lexical Tokens
Sieve scripts are encoded in UTF-8. The following assumes a valid
UTF-8 encoding; special characters in Sieve scripts are all ASCII.
The following are tokens in Sieve:
- identifiers
- tags
- numbers
- quoted strings
- multi-line strings
- other separators
Blanks, horizontal tabs, CRLFs, and comments ("white space") are
ignored except as they separate tokens. Some white space is required
to separate otherwise adjacent tokens and in specific places in the
multi-line strings.
The other separators are single individual characters, and are
mentioned explicitly in the grammar.
The lexical structure of sieve is defined in the following BNF (as
described in [ABNF]):
bracket-comment = "/*" *(CHAR-NOT-STAR / ("*" CHAR-NOT-SLASH)) "*/"
;; No */ allowed inside a comment.
;; (No * is allowed unless it is the last character,
;; or unless it is followed by a character that isn't a
;; slash.)
CHAR-NOT-DOT = (%x01-09 / %x0b-0c / %x0e-2d / %x2f-ff)
;; no dots, no CRLFs
CHAR-NOT-CRLF = (%x01-09 / %x0b-0c / %x0e-ff)
CHAR-NOT-SLASH = (%x00-57 / %x58-ff)
CHAR-NOT-STAR = (%x00-51 / %x53-ff)
comment = bracket-comment / hash-comment
hash-comment = ( "#" *CHAR-NOT-CRLF CRLF )
identifier = (ALPHA / "_") *(ALPHA DIGIT "_")
tag = ":" identifier
number = 1*DIGIT [QUANTIFIER]
QUANTIFIER = "K" / "M" / "G"
quoted-string = DQUOTE *CHAR DQUOTE
;; in general, \ CHAR inside a string maps to CHAR
;; so \" maps to " and \\ maps to \
;; note that newlines and other characters are all allowed
;; strings
multi-line = "text:" *(SP / HTAB) (hash-comment / CRLF)
*(multi-line-literal / multi-line-dotstuff)
"." CRLF
multi-line-literal = [CHAR-NOT-DOT *CHAR-NOT-CRLF] CRLF
multi-line-dotstuff = "." 1*CHAR-NOT-CRLF CRLF
;; A line containing only "." ends the multi-line.
;; Remove a leading '.' if followed by another '.'.
white-space = 1*(SP / CRLF / HTAB) / comment
8.2. Grammar
The following is the grammar of Sieve after it has been lexically
interpreted. No white space or comments appear below. The start
symbol is "start".
argument = string-list / number / tag
arguments = *argument [test / test-list]
block = "{" commands "}"
command = identifier arguments ( ";" / block )
commands = *command
start = commands
string = quoted-string / multi-line
string-list = "[" string *("," string) "]" / string ;; if
there is only a single string, the brackets are optional
test = identifier arguments
test-list = "(" test *("," test) ")"
9. Extended Example
The following is an extended example of a Sieve script. Note that it
does not make use of the implicit keep.
#
# Example Sieve Filter
# Declare any optional features or extension used by the script
#
require ["fileinto", "reject"];
#
# Reject any large messages (note that the four leading dots get
# "stuffed" to three)
#
if size :over 1M
{
reject text:
Please do not send me large attachments.
Put your file on a server and send me the URL.
Thank you.
.... Fred
.
;
stop;
}
#
# Handle messages from known mailing lists
# Move messages from IETF filter discussion list to filter folder
#
if header :is "Sender" "owner-ietf-mta-filters@imc.org"
{
fileinto "filter"; # move to "filter" folder
}
#
# Keep all messages to or from people in my company
#
elsif address :domain :is ["From", "To"] "example.com"
{
keep; # keep in "In" folder
}
#
# Try and catch unsolicited email. If a message is not to me,
# or it contains a subject known to be spam, file it away.
#
elsif anyof (not address :all :contains
["To", "Cc", "Bcc"] "me@example.com",
header :matches "subject"
["*make*money*fast*", "*university*dipl*mas*"])
{
# If message header does not contain my address,
# it's from a list.
fileinto "spam"; # move to "spam" folder
}
else
{
# Move all other (non-company) mail to "personal"
# folder.
fileinto "personal";
}
10. Security Considerations
Users must get their mail. It is imperative that whatever method
implementations use to store the user-defined filtering scripts be
secure.
It is equally important that implementations sanity-check the user's
scripts, and not allow users to create on-demand mailbombs. For
instance, an implementation that allows a user to reject or redirect
multiple times to a single message might also allow a user to create
a mailbomb triggered by mail from a specific user. Site- or
implementation-defined limits on actions are useful for this.
Several commands, such as "discard", "redirect", and "fileinto" allow
for actions to be taken that are potentially very dangerous.
Implementations SHOULD take measures to prevent languages from
looping.
11. Acknowledgments
I am very thankful to Chris Newman for his support and his ABNF
syntax checker, to John Myers and Steve Hole for outlining the
requirements for the original drafts, to Larry Greenfield for nagging
me about the grammar and finally fixing it, to Greg Sereda for
repeatedly fixing and providing examples, to Ned Freed for fixing
everything else, to Rob Earhart for an early implementation and a
great deal of help, and to Randall Gellens for endless amounts of
proofreading. I am grateful to Carnegie Mellon University where most
of the work on this document was done. I am also indebted to all of
the readers of the ietf-mta-filters@imc.org mailing list.
12. Author's Address
Tim Showalter
Mirapoint, Inc.
909 Hermosa Court
Sunnyvale, CA 94085
EMail: tjs@mirapoint.com
13. References
[ABNF] Crocker, D. and P. Overell, "Augmented BNF for Syntax
Specifications: ABNF", RFC 2234, November 1997.
[ACAP] Newman, C. and J. G. Myers, "ACAP -- Application
Configuration Access Protocol", RFC 2244, November 1997.
[BINARY-SI] "Standard IEC 60027-2: Letter symbols to be used in
electrical technology - Part 2: Telecommunications and
electronics", January 1999.
[DSN] Moore, K. and G. Vaudreuil, "An Extensible Message Format
for Delivery Status Notifications", RFC 1894, January
1996.
[FLAMES] Borenstein, N, and C. Thyberg, "Power, Ease of Use, and
Cooperative Work in a Practical Multimedia Message
System", Int. J. of Man-Machine Studies, April, 1991.
Reprinted in Computer-Supported Cooperative Work and
Groupware, Saul Greenberg, editor, Harcourt Brace
Jovanovich, 1991. Reprinted in Readings in Groupware and
Computer-Supported Cooperative Work, Ronald Baecker,
editor, Morgan Kaufmann, 1993.
[KEYWORDS] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[IMAP] Crispin, M., "Internet Message Access Protocol - version
4rev1", RFC 2060, December 1996.
[IMAIL] Crocker, D., "Standard for the Format of ARPA Internet
Text Messages", STD 11, RFC 822, August 1982.
[MIME] Freed, N. and N. Borenstein, "Multipurpose Internet Mail
Extensions (MIME) Part One: Format of Internet Message
Bodies", RFC 2045, November 1996.
[MDN] Fajman, R., "An Extensible Message Format for Message
Disposition Notifications", RFC 2298, March 1998.
[RFC1123] Braden, R., "Requirements for Internet Hosts --
Application and Support", STD 3, RFC 1123, November 1989.
[SMTP] Postel, J., "Simple Mail Transfer Protocol", STD 10, RFC
821, August 1982.
[UTF-8] Yergeau, F., "UTF-8, a transformation format of Unicode
and ISO 10646", RFC 2044, October 1996.
14. Full Copyright Statement
Copyright (C) The Internet Society (2001). All Rights Reserved.
This document and translations of it may be copied and furnished to
others, and derivative works that comment on or otherwise explain it
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