Have an issue that is more of a puzzle than a problem. No real harm done. So don't spend much time on it if you don't recognize the issue.
I am administrator for a small list, radg@lists.radg.us, hosted by Mailmanlists.net. They are trying hard to figure it out but so far unsuccessful.
Looking at the headers of emails sent from steve@stevebrown.us to the list and boomeranged back to me, I see a line “X-MailFrom: bounce+66273f.99dcd9c-radg=lists.radg.us@stevebrown.us”. I gather that it is designating the return path where a message would be sent if there was a delivery problem. From its position in the header, I suspect it is added as they pass through Mailgun, maybe related to DKIM. I am hoping you might know why that bounce address is recognized by Mailman 3 as a real address for my emails, but not for others, even from my wife’s computer traversing the same path from our home to the Mailman 3 server. The Mailman installation does get and process the post, but also puts the “bounce+,,,” address in its list of nonmembers. Mailmanlists.net hasn’t figured out the behavior. I can provide full header information if that would help. My understanding of how headers work is very limited.
Steve Brown