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.