Genius. Yes, the bodyhash_mismatch is in the DKIM stuff, and the message returned from MM3 has a footer attachment. Although I had modified the default to contain no content, it is still 129 bytes.
Second, the reason why I saw different behavior from my posts and my wife's was that she wasn't a member. When I made her a member and sent another post from her address, the bounce+ address showed up in the nonmember list identically. So all of this has something to do with how our email host handles the outgoing mail. TMD uses Mailgun as a mail handler that provides deliverability services. I have a ticket in to them, but haven't heard back.
Mailgun appears to modify the return path to a bounce+ address that MM3 sees as a nonmember sender, and then the recipient's email handler sees it as equivalent to another address that might be in the envelope. The header of a message sent from steve@stevebrown.us to my radg.us address shows:
spf=pass (fr-int-smtpin1.hostinger.io: domain of "bounce+906d23.99dcd9c-administrator=radg.us@stevebrown.us" designates 143.55.232.17 as permitted sender) smtp.mailfrom="bounce+906d23.99dcd9c-administrator=radg.us@stevebrown.us";
I think I have bothered you enough for something that does not seem to create any real problems or threaten to do so. So reply only if something here stimulates another thought.