After posting a message to this list earlier today, I immediately received nearly a dozen DKIM fail messages, all being sent by the "OpenDMARC Filter" at various domains, and all saying that the DKIM fail reason was "signature verification failed".
I raised this issue with support at my hosting company, who pointed out that a) my domain's DKIM records are set up correctly, and b) one of the headers on my email that got sent to the list indicated that Arch's mailman actually did successfully verify my domain's DKIM record when it received my email, before forwarding it on.
Anyone know why these fail messages might be happening? Is this being caused by a misconfiguration in Arch's mailman installation? Or is this a misconfiguration of the individual OpenDMARC software installations at each of those domains?