I don't know about Microsoft, but I get a couple of DMARC reports per day from Google's email infrastructure. I also get reports from aol.com, yahoo.com, linkedin.com, comcast.com, fastmail.com, etc.
It was sending an HTML email with the appspot domain used for the logo: https://example.appspot.com/images/logo.png
Changing that to http://www.example.com/images/logo.png fixed the problem, but now I have to host the image elsewhere on a HTTPS enabled domain.
A separate question is what has google done with all the messages that users of this app sent? Are they gone for good, or stuck in a queue somewhere?