What is SIEVE?

SIEVE is a programming language designed for users to configure mail filters on mail servers.

Sieve was created to allow users to easily define their own rules for filtering mail. However, it is not possible to use more complex program structures, such as those that use loops or variables, or to run external programs.

Sieve scripts are already executed during the delivery of the e-mail at the incoming mail server.

See also:
Wikipedia: SIEVE