maildrop (mail delivery agent with filtering abilities)

maildrop is the mail filter/mail delivery agent that's used by the
Courier Mail Server.  This is a standalone build of the maildrop mail
filter that can be used with other mail servers.

maildrop is a replacement for your local mail delivery agent. maildrop
reads a mail message from standard input, then delivers the message to
your mailbox. maildrop knows how to deliver mail to mbox-style
mailboxes and maildirs.  "maildir" is a mailbox format used by the
Courier mail server and Qmail.