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
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   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
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   Message B
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   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!
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