What is the crowd's opinion of
Trojitá?
From 3.5 billion comments
2 reviews of this app found across the web:
That's the one. It also has an official webpage: https://kube.kde.org/
AFAIK KMail and Akonadi will continue to be developed at least until it can actually be replaced with something mature.
Kube and Sink are still in early stages, but also have somewhat different personas/use cases in mind.
If you simply need an fast IMAP mail client, check out Trojita: http://trojita.flaska.net/
I don't have any issues running Thunderbird, honestly. But it is gtk. Here's Thunderbird's feature list. Also, here's a screenshot of it running on my system so you can get an idea of how it looks in the environment (in case you've somehow never used it before?).
I've also heard good things about Geary, Claws Mail, and (of course) Evolution but again, those are all gtk.
In the Qt world, I know Trojitá exists, but I otherwise I'm completely unfamiliar.
If you're looking for something specifically built for and targeted to KDE, I don't know of anything other than KMail.