This program is excellent as is, and with a little knowledge of php, it is VERY customizeable.
Hate it!! I actually changed my hosting company because they forced us to use SquirrelMail... The UI is terrible.. Delete buttons on the right on one page, then moved the the left on another. The text is ungodly ugly. The navigation is terrible, the options are soooooo limited.
There are no 4 star reviews.