It seems there is a problem in the communication with Akonadi, which makes Kmail to fail in loadin emails. Then after some time trying to load them, Kmail sends that message and crashes.
perhaps you can give us more info :)
For example info from console etc.
We don't have enough information to help you
I tried to reproduce the issue Today and couldn't (I upgraded from 16.08.3 to 16.12 again). So it might be a packaging issue that got somehow fixed, perhaps? I'll keep version 16.12 and I will upload here the output from the terminal if the issue reappears at some point...
I think this was a package bug. I didn't experience any more problem after neon updated kdepim