The mail filter/mail delivery agent from the Courier Mail suite.
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 Courier and Qmail.
maildrop optionally reads instructions from a file, which describe how to
filter incoming mail. These instructions can direct maildrop to deliver the
message to an alternate mailbox, or forward it somewhere else. Unlike procmail,
maildrop uses a structured filtering language.
maildrop is written in C++, and is significantly larger than procmail. However,
Sam Varshavchik <mrsam at courier-mta dot com>