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RFC3028 - Sieve: A Mail Filtering Language

王朝other·作者佚名  2008-05-31
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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 RFCnumber:

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 RFCnumber:

RFC3028 (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 RFCnumber:

RFC3028 (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 RFCnumber:

RFC3028 (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 RFCnumber:

RFC3028, Sieve, by reference of

RFC2244, 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 RFC2048 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 RFC3028.

Interoperability considerations: Discussed in section 2.10.5

of RFC3028.

Published specification: RFC3028.

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 RFC3028.

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", RFC2234, November 1997.

[ACAP] Newman, C. and J. G. Myers, "ACAP -- Application

Configuration Access Protocol", RFC2244, 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", RFC1894, 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, RFC2119, March 1997.

[IMAP] Crispin, M., "Internet Message Access Protocol - version

4rev1", RFC2060, December 1996.

[IMAIL] Crocker, D., "Standard for the Format of ARPA Internet

Text Messages", STD 11, RFC822, August 1982.

[MIME] Freed, N. and N. Borenstein, "Multipurpose Internet Mail

Extensions (MIME) Part One: Format of Internet Message

Bodies", RFC2045, November 1996.

[MDN] Fajman, R., "An Extensible Message Format for Message

Disposition Notifications", RFC2298, March 1998.

[RFC1123] Braden, R., "Requirements for Internet Hosts --

Application and Support", STD 3, RFC1123, 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", RFC2044, 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

or assist in its implementation may be prepared, copied, published

and distributed, in whole or in part, without restriction of any

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