Howtoforge posted a new Debian guide: Virtual Users And Domains With Postfix, Courier And MySQL

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 will 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 And MySQL (Debian Etch)