Sieve Extension: Relational Tests

Sieve [SIEVE] is a language for filtering e-mail messages at the 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 Messages Access Protocol (IMAP) servers, as it has no variables, loops, nor the ability to shell out to external programs.