This is the first problem I see with Kmail 16.12 after updating Then restarting Akonadi and Kmail trying to fix the problem makes the bug https://bugs.kde.org/show_bug.cgi?id=373832 to appear, and from there on there is no way to work with Kmail properly in my system that does not imply coming back to version 16.08.3
I get this message in the terminal when trying to unsuccessfully send an email: org.kde.pim.akonadicore: Got a stale notification for an item which was already removed. 327594 ""
This issue and https://bugs.kde.org/show_bug.cgi?id=373832 might be packaging issues (?). At least this issue can be somehow workarounded now after some combination of: - manually stopping the Akonadi server - reinstalling the Akonadi agent in charge of sending emails - restarting Kontact
If it works after reinstalling package, yep it seems that it's a packaging problem.
I think this was a package bug. I didn't experience any more problem after neon updated kdepim