fdm is a program to fetch mail and deliver it in various ways
depending on a user-supplied ruleset. Mail may be fetched from stdin,
IMAP or POP3 servers, or from local maildirs, and filtered based on
whether it matches a regexp, its size or age, or the output of a shell
command. It can be rewritten by an external process, dropped, left on
the server, or delivered into maildirs, mboxes, to a file or pipe, or
any combination.

fdm is designed to be lightweight but powerful, with a compact but
clear configuration syntax. It is primarily designed for single-user
uses but may also be configured to deliver mail in a multi-user setup.
In this case, it uses privilege separation to minimise the amount of
code running as the root user.