I live in the KDE ecosysteem so I gave Kmail a serious shot, but it is such a pain to setup with multiple accounts that I gave up. It is very easy to misconfigure sth that I can't think of being a use case anybody needs, but then things like reply to all by default were removed because it ahem a certain group of FLOSS Devs is against such shenanigans. Pity, because apart from these issues I really like this mua.

Thunderbird is and will be for some time the only realistic least bad option. Once they replaced xul, perhaps updating its visuals will get easier.