If KMail is closed or crashes then it restores the message windows which were open in that moment. This is an extremely useful feature. Because I often have so many message windows open (those which I have to reply to) that I easily lose track, I have put some of those windows on the next virtual desktop. These windows are not restored after quitting and restarting KMail. The behaviour is the same if the system crashes. Reproducible: Always
This bug has never been confirmed for a KDE PIM version that is based on KDE Frameworks (5.x). Those versions differ significantly from the old 4.x series. Therefore, I plan to close it in around two or three months. In the meantime, it is set to WAITINGFORINFO to give reporters the oportunity to check if it is still valid. As soon as someone confirms it for a recent version (at least 5.1, ideally even more recent), I'll gladly reopen it. Please understand that we lack the manpower to triage bugs reported for versions almost two years beyond their end of life.
Just as announced in my last comment, I close this bug. If you encounter it again in a recent version (at least 5.1 aka 15.12, preferably more recent), please open a new one unless it already exists. Thank you for all your input.