maildrop is a replacement for your local mail delivery agent. It 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. maildrop will optionally read instructions from a file, which describes how to filter incoming mail. Instructions can be provided having mail delivered to alternate mailboxes, or forwarded somewhere else. Unlike procmail, maildrop uses a structured filtering language. maildrop is written in C++, and is significantly larger than procmail in compiled form. However, it uses resources much more efficiently. Unlike procmail, maildrop will not read a 10 megabyte mail message into memory. Large messages are saved in a temporary file, and are filtered from the temporary file.
WWW: http://www.courier-mta.org/maildrop/