Since update of kmail to 5.8 the standard domain inside the identity-> tab advanced (Erweitert) does not work. 1. Set standard domain in your identity 2. write a new e-mail with only the user of the standard domain 3. send e-mail -> e-mail turn gray and stop working, it couldn't be saved to draft directory. Happens all the time. Hope, it will help you.
It is now working for quite some time.