Oh, follow-up (and this, too, is for Devuan's listadmins' attention in
particular):

> There is an (IMO unhappy but least-bad-available) kludge setting in
> Mailman's admin WebUI to make the MLM compensate for DMARC brain-damage:
> You go to Privacy Options, Sender Filters, item 'Action to take when
> anyone posts to the list from a domain with a DMARC Reject/Quarantine
> Policy' aka dmarc_moderation_action. Change the radio button from
> Accept (default) to Munge from.

I am specifically _not not not_ recommending the similar-looking setting
'Replace the From: header address with the list's posting address to
mitigate issues stemming from the original From: domain's DMARC or
similar policies' aka from_is_list on General Options. My understanding
is that opting for _that_ version of the kludge unconditionally applies
it to all postings whether they are from DMARC-encumbered domains or
not.

My recommendations:

On Privacy options, Sender filters:
dmarc_moderation_action: Munge from
dmarc_quarantine_moderation_action): Yes
dmarc_none_moderation_action: no

On General options:
change nothing.