Thanks, much easier to communicate when runnable code is involved :) Now I can see what you mean about it writing the From.  I will figure out why and fix it so that the From line is not written.

The traceback from email.generator is unfortunate.  I should have message_from-string reject non-ASCII input with a clear error message like mailbox.add does now, but I didn't think of it.  I will see if the release manager will let me make that change.

For case (2) it is working as designed: faithfully writing the non-conformant message to the mbox.  If you wish to do other sorts of handling of non-conformant email you need to write code to do that.  You can use the email package to make it technically conformant (and ASCII only) by doing something like message_from_string(str(message_from_bytes(non_conformant_message)))  I'm not sure why you'd want to do that, though :)