Thunderbird (in German) uses the charset
-1, can't check right now) by default for outgoing and incoming mails. (If you want to change the default, see Change Thunderbird default encoding?)
I wonder why TB doesn't use
UTF-8 as default. So I searched for it and the only reason I found was, that some older mail servers and clients couldn't handle UTF-8. I read the issue (closed as "WONTFIX"), but couldn't find a concrete list which servers/clients/providers would have problems handling UTF-8.
Will you get problems if you send your mails UTF-8 encoded today?