I think I’ve been stung by the same server twice. I vaguely recall another address that resolved to barracudanetworks.com, where msgs from that other user to me had MS outlook in the headers. So I wonder if barracudanetworks is setup to be a proxy or façade of sorts for MS, in which case I could track this.

So what if for every email I receive, I have a program that checks for MS in the headers and (if MS matches) it does an MX lookup on the sender address, which it could then store in a DB to track patterns. This could also be a shared DB so a group of people could benefit from associating non-MS MX servers (like barracudanetworks) to MS. Imperfect of course, but perhaps accurate enough?

Ultimately this is GDPR issue. Data subjects are supposed to be able to know who gets their info and legally they have control over it. Email is somewhat incompatible with the GDPR in this regard.