Thank you guys for explanations and sharing your experience.
You've just proved the worst thing I could think of, unfortunately

. Either the new Kmail is good and easy-to-handle or total crap - I'd have an answer to that: keep it or abandon it forever. But here I got stuck: if it is almost OK and I manage to import my e-mail database, where is the guarantee that the next time I upgrade (to Mageia 5 or whatever it will be) it will not cause severe problems like in KDE4.8, but this time with more e-mails accumulated? It is not the question I am addressing anyone, just reasoning

. In addition to that, AFAIK, Qt5 and KDE5 are already somewhere within reach, so I expect the same problems as we had experienced with transition to KDE4...
So right now I am leaning to trying Thunderbird (or something from Gnome) and if I find it stable enough, I'm afraid, I'll have to leave KDE as I have pretty bad experience with it after switching from 4.6.5 to 4.10. It's a pity of course, mostly because of the style I like and a set of application I've been using for years, but what's the point to keep using them if they are getting more and more buggy?

Once again, thank you.