There's also IceWarp that we're currently using to support about 800 users. Their Windows side is better than Linux, but Linux is coming around and getting better. They have their own desktop email client, webmail, jabber ability, and groupware.
Check them out and if you're interested I can you in touch with some management folks there that we communicate with daily. (I don't work for IceWarp, we are a customer).
Website: IceWarp