for a long time, thunderbird + enigmail offer a feature to move certain mail headers to the encrypted mail body. that way, it was possible to hide information like the mail subject (it's replaced by "..."). with current versions of thunderbird 78, this feature became mandatory for OpenPGP encrypted mails and cannot be disabled. this currently can create a mess in your inbox.


STEPS TO REPRODUCE
1. have someone send you an OpenPGP encrypted mail using thunderbird >= 78

OBSERVED RESULT
kmail shows the subject as "...". when the mail is decrypted, you find the missing headers at the top of the mail body. in grouped folder view, all messages sent by thunderbird users form one huge thread with subject "...", it is literally impossible to quickly find mails or even a particular mail thread.

EXPECTED RESULT
kmail should be able to replace the hidden subject with the actual one. it should also be able to keep the memory hole format in replies.

ADDITIONAL INFORMATION
i personally wouldn't want to use such a feature. but in order to be able to communicate with people who are unable to disable it, kmail should support it so that you can get human readable subject lines back.

here's the spec: https://github.com/autocrypt/memoryhole/blob/master/specs/draft-memoryhole.md