I've now disabled all content filtering in Mailman, because of the failing DKIM/DMARC issues. Hopefully that solves them for now.. Although this will mean that we'll probably start getting HTML mails to this list, which isn't ideal. I guess an alternative could be to reject any mails with text/html MIME part, but that could prevent some people from sending any mails to this list unless they switch to a different client/webmail.. Also that feature doesn't seem to be possible at least with the Mailman version we're currently using.
BTW. All/most of the Gmail and Yahoo users just got disabled by mailman due to DMARC bounces. I've (hopefully) re-enabled mail for all of them.