The maildrop Mail Delivery Agent (MDA) also includes filtering functionality.
Maildrop receives mail via stdin and delivers in both Maildir and mbox formats.
Maildrop can optionally read filtering configuration from a file to determine whether the message is delivered to an alternate mailbox or forwarded.
Running in embedded mode allows a filter to reject a message before the server accepts it for local delivery.
However, it uses resources more efficiently than procmail (written in C) by saving large messages to a temporary file rather than reading them into memory.