I wrote:
I am not familiar enough with DMARC and Mailman3 to know whether there are any work-arounds available for this issue or not. There aren't any great solutions to it, in any case.
A little further reading tells me that Mailman supports a few possible work-arounds¹. I've mentioned these to the list admins and will see if any of them are worth testing².
¹ https://wiki.list.org/DEV/DMARC ² Only 'Replace From:' or 'Wrap message' would be reasonable for this list, IMO. If it were @aol.com causing this trouble, I'd be more inclined to suggest the 'Discard' option. ;)