SUMMARY
After every tool start the message encoding (for the window displaying the selected message) in menu Ansicht/Kodierung festlegen... (Germanversion; I would expect something like View/Encoding) is set to "Baltisch" ("Baltic"). i always have to set it to "Automatisch" ("Automatic") manually.
At the next restart (after rebooting) the setting is lost again.

I could not find another setting to det the enclding besides the mentioned menu entry.

STEPS TO REPRODUCE
1. Start kmail

OBSERVED RESULT
Encoding is set to Baltic

EXPECTED RESULT
Encoding is set like it was during closing, i. e. "Automatic" in my case.

SOFTWARE/OS VERSIONS
Windows: 
macOS: 
Linux/KDE Plasma: 
(available in About System)
KDE Plasma Version: 
KDE Frameworks Version: 
Qt Version: 

ADDITIONAL INFORMATION