Funny this should come up now: I just recently moved away from Mutt to Thunderbird. My online university technically requires OE, but I've had no troubles using TB instead -- the only reason Mutt didn't work is that HTML-mail is required by the University.
I have this strange compunction to read all my mail with the same app, so I've moved over to TB entirely. Long live IMAP for making that painless!
"Users are evil. All users are evil. Do not trust them. Perl specifically offers the -T switch because it knows users are evil." - japhy