Our messaging group looked at this and is going to lessen the header size by adjusting some filtering available in Office 365. What appeared to have happened was Microsoft developed a new enthusiasm for email diagnostics and spam prevention.  The newer products became header heavy without an awareness of the impact on other email systems. My guess at least.

 

I was looking thru some of the Transport Rule options and I might have found a solution.

It appears I am able to strip out individual header entries

Now the challenge is to figure out which header entries are huge and mostly unnecessary