Quoting Hendrik Boom (hendrik@???):

> I hope it doesn't add a reply-to header if there's already one.

I can't say for certain, but it's likely GNU Mailman would (in that
case) add the poster's real address as an additional address within the
existing Reply-To: address (if they even differ).

I thus speculate because tat's the heuristic I see modern versions of
GNU Mailman follow when the listadmin has enabled conventional
(non-DMARC related) Reply-To: munging (the type infamous for triggering
Internet flamewars, the munging practice obsoleted by IETF in 2001 via
RFCs 2822 and 2369, but still popular on mailing lists catering to
technophobes). So, I would expect Mailman's DMARC-mitigation Reply-To:
addition to use the same code.