A G Suite account was the first thing that I thought of as well, but no, this is straight gmail.com. I actually tested with my personal G Suite account, and I got similar results. Because I’m in the support team for the manufacturer, we’ve been hearing reports from a large number of customers who have been complaining of this over the past 48 hours.
That’s a great suggestion, but the message gets tossed by the M365 spam filters before it reaches the inbox and I never have a chance to see the headers. All I’m able to see is the snippet of information that doesn’t show anything actionable.