DBMail - MDA written in C by Eelco van Beek and Roel Rozendaal. Provides IMAP and POP3 daemons that use a MySQL or PostgreSQL mail store. http://www.dbmail.org/
dystill - A filtering MySQL database driven mail delivery agent, http://www.dystill.org/
Cyrus IMAP - A mail server suite that includes a mail delivery agent (but note that it is limited to storing mail in the bespoke mail store)