Sieve
sieve.info
From the page
Saved with the link. Notes from 2007 to 2016 are mostly the page's own words, so this is probably an excerpt.
Sieve (RFC 5228) is a language for filtering e-mail messages. 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 IMAP servers, since in its basic form it has no variables, loops or ability to shell out to external programs.