* Stephen J. Turnbull:
It's possible to do this only for domains that advertise p=reject. They deserve what they get.
If there are any issues that seem like they can be addressed in upstream GNU Mailman (unfortunately, we don't carry a stick big enough to convince blockheaded mail domains not to publish p=reject), let me know and I'll push it with the Mailman devs. No promises, of course.
Gmail *recipients* (which includes most redhat.com subscribers these days) need this rewriting as well. Discarding mail is always a policy decision carried out by the recipient, so it needs to be configurable for mailing list subscribers. The mailing list server cannot detect this recipient behavior automatically.
The DMARC policy set by the sender does not really matter here. People tell you differently, but they are misinformed because the discard policy is implemented by the recipient, not the sender.
Thanks, Florian