Howtoforge posted a guide about setting up virtual users and domains with Postfix, Courier, MySQL and SquirrelMail on Fedora 13

Virtual Users And Domains With Postfix, Courier, MySQL And SquirrelMail (Fedora 13 x86_64). This document describes how to install a mail server based on Postfix 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.

  Virtual Users And Domains With Postfix, Courier, MySQL And SquirrelMail (Fedora 13 x86_64)