Howtoforge published a guide about setting up virtual user and domains with Postfix, Courier, MySQL, SquirrelMail under Fedora 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. 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 (quota is not built into Postfix by default, I'll show how to patch your Postfix appropriately). Passwords are stored in encrypted form in the database (most documents I found were dealing with plain text passwords which is a security risk). 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 (Fedora 10)