Howtoforge posted a guide about installing a Postfix mail server on CentOS 4.8

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 used by Postfix.

Virtual Users & Domains With Postfix, Courier, MySQL & SquirrelMail (CentOS 4.8)