Message/EmailMessage are intentionally mimicing the full dictionary API, not just the Mapping API (with a couple exceptions and several extensions).  But it is important to mention that it is ordered, both for the reason INADA stgtes, and because the text goes on to discuss the handling of duplicate headers and what happens when you add and delete headers.  While this turns out to be almost the same as the rules used by a standard python dictionary, that is partially a coincidence....other ordering rules could have been chosen.