Running latest kmail (5.6.0) with plasma 5.10.5 on ArchLinux. Every time I log out of plasma while kmail is running, it crashes. It blocks the logout process until the crash windows gets force closed after a few seconds. Happens in 100% of all cases. It's kind a hard to debug, because the crash window gets killed automatically after a few seconds. If you know of a way to get the crash report nevertheless, please tell me. I can't even copy paste out of it, because I can't paste it anywhere.
I can confirm this bug. Kmail always crashes when logging out of Plasma. Up-to-date packages on Arch Linux.
*** This bug has been marked as a duplicate of bug 378315 ***