I like the convenience of Squirrel Mail very much. It's great toa be able to get my email from anywhere with a minimum of fuss and bother.
I ESPECIALLY like the ability to assign a color to emails from various other senders -- I wish all email programs had this capability.
One change I'd suggest is to put all of the menu optons in the left side frame statically, so they are available to the user regardless of where the user is in the body of an email. For example, Next, Previous, Delete, Message List, etc. All menu options should be in one place.