Howtoforge posted a guide about setting up virtual users/domains with Postfix, Courier, MySQL and SquirrelMail under Ubuntu 8.10

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. Passwords are stored in encrypted form in the database. In addition to that, this tutorial covers the installation of Amavisd, SpamAssassin and ClamAV so that emails will be scanned for spam and viruses. I will also show how to install SquirrelMail as a webmail interface so that users can read and send emails and change their passwords.

Virtual Users/Domains With Postfix, Courier, MySQL & SquirrelMail (Ubuntu 8.10)