Sure... anything that supports IMAP (or POP3) should work (it's gotta access the mail somehow). Note that we don't have IMAP or POP3 support built into our versions of PHP. This just means that programs which rely on PHP's built in IMAP / PHP functionality won't work.

Personally, I think squirrelmail is better than most other webmail systems I've seen / used (though I detest webmail and rarely use it at all for anything).