Dovecot is a secure and compact IMAP server which is in the early stages
of development. It supports Maildirs and mbox formats and much of the
IMAP v4 protocol including SSL/TLS. IPv6 support is also included.
Dovecot supports authentication with OpenLDAP, PostgreSQL, MySQL, vpopmail
and PAM.

WWW: http://www.dovecot.org/