This document describes a language for filtering email 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 the base language has no variables@ loops@ or ability to shell out to external programs.