SUMMARY

Since a few weeks, kmail has stopped to display special characters correctly in  incoming emails sent via Exchange servers. This happens independently of the specific choices under "View/Set encoding". "Auto" should do the job, but fails. Any argument that the incoming email probably has a wrong encoding in its header is NOT applicable: 
1) any other email client can handle the same emails correctly without any fiddling;
2) most important and annoying: it worked perfectly until January/February 2020 or so - but then the updates started to be buggy!

Interestingly, the encoding becomes correct when forwarding/replying to the email.

This is bug should definitely not occur in modern email client in 2020.

Please amend - otherwise kmail is definitely no longer usable! 

STEPS TO REPRODUCE
1. Get an email from an Exchange account 
2. View text.
3. See wrong encoding.

OBSERVED RESULT

Wrong umlauts/special characters in German. 

EXPECTED RESULT

Correct encoding - as accomplished by any modern email client.

SOFTWARE/OS VERSIONS
Windows: 
macOS: 
Linux/KDE Plasma: Tumbleweed (status: 23/03/2020)
(available in About System)
KDE Plasma Version: 
KDE Frameworks Version: 
Qt Version: 

ADDITIONAL INFORMATION

1) New bug - encoding worked perfectly until January/February 2020, definitely in 2019!
2) Encoding gets correct in forwarded/replied emails!