One additional piece of information, that is specific to Outlook.com, is this, from another group administrator responding to my complaint that the reason for the bounce is not obvious:

It would be if Outlook didn't insert all that extra crap up front.  Other email services don't add it, so the error is right at the top.

So all the gobbledy-gook that Outlook inserts ahead of the line that starts with, "Diagnostic information," are only Outlook, and they make the situation more confusing, not less.
--

Brian - Windows 10, 64-Bit, Version 21H1, Build 19043  

Science has become just another voice in the room; it has lost its platform.  Now, you simply declare your own truth.

       ~ Dr. Paul A. Offit, in New York Times article, How Anti-Vaccine Sentiment Took Hold in the United States, September 23, 2019