We offer our clients SquirrelMail. As was said above, the development is still very active and there are tons of plugins available to enhance the user experience. Our customers prefer it very much over Horde, which we offered before switching to SM.
Can't help you any more here. SquirrelMail and the plugins available for it met my needs and the needs of all of my customers. I beta tested it with a few customers who loved it, so I just went with SquirrelMail. I have never used the others.
. If you want to provide a Hotmail like interface to your customers and price doesn't matter, try MailBee Pro v3.1. Don't use 3.0, as it would not delete emails from server when you remove them from trash.
. Try free hMail Server
. Try DWMail. Lowest in price and cuttest in GUI.