Maildrop is a combination mail filter/mail delivery agent.
Maildrop reads the message to be delivered to your mailbox,
optionally reads instructions from a file how filter incoming
mail, then based on these instructions may deliver mail to an
alternate mailbox, or forward it, instead of dropping the
message into your mailbox.

Maildrop uses a structured, real, meta-programming language in
order to define filtering instructions. Its basic features are
fast and efficient. At sites which carry a light load, the
more advanced, CPU-demanding, features can be used to build
very sophisticated mail filters. Maildrop deployments have
been reported at sites that support as many as 30,000
mailboxes.

Maildrop mailing list:
http://lists.sourceforge.net/lists/listinfo/courier-maildrop

This version is compiled with support for GDBM database files,
maildir enhancements (folders+quotas), and courier authlib
support.