SUMMARY
When I am composing an e-mail, or replying to an existing one, the To: and CC: fields are converted to BCC:. This is very annoying and it makes kmail kind of unusable.

STEPS TO REPRODUCE
1. Create a new e-mail in kmail (or kontact)
2. Type an address in the To: field
3. Type a second address in the second address field. Change it from To: to CC:
4. Type a subject and an email contents
5. Send the email

OBSERVED RESULT
The email gets sent, but the recipient addresses are randomly (IMO) moved into BCC. It does not seem to depend on the addresses I use, I have done several tests with the same two addresses and sometimes one of them is converted to BCC, sometimes both.

EXPECTED RESULT
The To: and CC: recipients are kept and the email is sent correctly, nothing is put into BCC (unless I manually include bcc recipients).

SOFTWARE/OS VERSIONS
Fedora 33, KDE 5, kmail 5.15.1

ADDITIONAL INFORMATION
I'm testing sending email from my corporate Gmail address, which works perfectly using Evolution (so I do not think the issue is on the mail server side). If any test/logs/files are needed, please let me know.