I used to use claws mail, in fact for a while I hosted and built the rpms for fedora (until it was merged and kept up to date by the fedora team)

But for me now it isn't usable at all now, and only kmail is usable.
Email clients are very very subjective view and it depends what you want, also people moan very loudly - but keep quiet when they like something (except for Apple of course, where you aren't allowed to not like it!)

Modest for me is fine, it's not the greatest and it is missing functionality - which is making me think of developing for it, but it is fairly nice, and getting better all the time.