Howtoforge posted a tutorial about installing an Ubuntu 12.04 server with Postfix, Courier, MySQL, and SquirrelMail

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, 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.

  Virtual Users And Domains With Postfix, Courier, MySQL And SquirrelMail (Ubuntu 12.04 LTS)