Not that Thunderbird is a bad choice!

There are several good alternatives. The new Kmail based on QT4 is great. If you
want speed and flexibility, Claws is a great choice and it is GTK. Both Claws
and Kmail have many plugins to integrate with other programmes (Spamassassin or
Bogofilter for example). Kmail has full html support, but I am not sure about
Claws.