This document describes how to install a Postfix mail server that is based on virtual users and domains, i.e. users and domains that are in a MySQL database. I'll also demonstrate the installation and configuration of Courier (Courier-POP3, Courier-IMAP), so that Courier can authenticate against the same MySQL database Postfix uses. The resulting Postfix server is capable of SMTP-AUTH and TLS and quota. In addition to that, this tutorial covers the installation of Amavisd, SpamAssassin and ClamAV so that emails will be scanned for spam and viruses.

http://www.howtoforge.com/virtual_postfix_mysql_quota_courier_ubuntu_edgy