Sieve mail filtering language

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SIEVE email filtering language

Sieve is 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 RFC822-compliant messages, but otherwise should generalize to other systems that meet these criteria.

The language is powerful enough to be useful, but limited in power 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 GUI-based editors. The language is not Turing-complete, and provides no way to write a loop or a function. Variables are not provided.

Implementations of the language are expected to take place at time of final delivery, when the message is moved to the user-accessible mailbox. In systems where the Mail Transport Agent (MTA) does final delivery (such as with traditional UNIX mail systems) it is reasonable to sort when the MTA deposits mail into the user's mailbox. However, Sieve filters might be used at a number of "final delivery" points in a mail system: by an SMTP server, by an IMAP or POP server filing into one or more mailboxes, or by a Mail User Agent (MUA or mail client) acting as a delivery agent (for instance, a POP or offline IMAP client.)

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 most users.