RFC 3028 – Sieve: A Mail Filtering Language

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.