Howtoforge posted a tutorial about setting up virtual users and domains with Postfix, Courier, MySQL, and Squirrelmail on Ubuntu

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. Passwords are stored in encrypted form in the database. In addition to that, this tutorial covers the installation of Amavisd, SpamAssassin and ClamAV. 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 11.10)