Flavio wrote (2022-02-22):

 FB> On 21 Feb 2022, Tommi Koivula said the following...

 TK>> All I did was to add these lines:

 TK>> read UTF-8      utf-8
 TK>> post utf-8      UTF-8


 TK>> Which Thunderbird are you using? The newest TB seems to always post
 TK>> messages in utf-8. Annoying...

 FB>      Thanks, I will give it a try.

That should work. I have the same configuration and no problems with posting UTF-8 messages.

 FB>      Exactly. That started to happen after the latest TB upgrade for
 FB> Mac.

 FB>      I checked other news clients and it worked fine...

Looks like this 'feature' has been introduced by Firefox:

https://old.reddit.com/r/Thunderbird/comments/ps6gxp/is_text_encoding_options_gone/

I haven't used the newer Thunderbird versions. Does it always use UTF-8 even if the text in the mail is only 7-bit ASCII?