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.