I believe the OP can get exactly the effect he wants by specifying a DMARC policy record including "p=quarantine; pct=0". Mailman doesn't (and arguably shouldn't) check the value of pct, let alone roll dice, on p=quarantine or p=reject, so this policy will cause Mailman to mitigate (if mitigation is available in that version of Mailman and requested by the list admin). On the other hand, pct=0 means no conforming recipient implementation will actually quarantine any messages. (Yes, pct=0 is explicitly permitted by the RFC.) The vast majority of mail is covered by conforming implementations according to DMARC.org testing (but I don't know that this corner case was tested -- someone with a test domain can do that easily enough, and if someone does, please report whether it worked and which providers were tested to mailman- firstname.lastname@example.org).