Howtoforge published a guide about setting up virtual users and domains on Ubuntu Linux 8.04

This document describes how to install a Postfix mail server based that is based on virtual users and domains, i.e. users and domains that are in a MySQL database. I will 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. 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 And Domains With Postfix, Courier, MySQL And SquirrelMail (Ubuntu)