Antimatter Studios

Postfix SMTP/SMTPD server used to send and receive emails from the outside world

A dovecot server to deliver email to clients, authenticate them, and send emails

The database which stores all the accounts, dkim keys, etc

The DKIM project which can check your domain keys against a database of recognised entries

A RspamD server to weed out spam sent to the mail server

A ClamAV Service which can scan emails for viruses