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.