Ah, yes, that would do it.  EmailMessage should definitely throw an error if it gets compat32 as a policy, that would have told you what you were doing "wrong".

What you want to be doing is:

  msg = message_from_string(mystring, policy=default)

That will get you an EmailMessage object.  Note, however, that you almost never want message_from_string, because real email messages are bytes objects, not strings.  string only works if your message is ASCII only. (There's an open issue about documenting this properly...and a big doc revision in the offing.)