[Skunkworks] Webmail apps

Good people, I am in search of an application that can handle webmail for about 1000 users; open source, web based, preferably done with PHP and storing all data in a MySQL database. It is intended to be deployed in an intranet. RoundCube <http://roundcube.net/> seems like a good option, anyone who has used it? How is it? What other reliable apps are available?