maildrop is the mail filter/mail delivery agent that’s used by the Courier Mail Server. You do not need to download maildrop from here if you already have the Courier mail server installed. 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.

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, 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. If the standard input to maildrop is a file, and not a pipe, a temporary file will not be necessary.

maildrop checks the mail delivery instruction syntax from the filter file, before attempting to deliver a message. Unlike procmail, if the filter file contains syntax errors, maildrop terminates without delivering the message. The user can fix the typo without causing any mail to be lost.