Comment Claws-mail (Score 3, Informative) 464
Claws-mail is the successor of the old Sylpheed-claws. It really is a nice and simple mailclient, which in the meantime does almost everything. Imaps, RSS, filtering, whatever. And with good usability, the buttons are all at the right place.
I even use the Windows version at work.
There are some thing Thunderbird is particularly bad at in my opinion. Like sorting threaded mails. I know there are extensions, but they suck.
I also don't like the autodetection of mailserver settings. You cannot save something in a non-working state, while sometimes I just want to do that.