tomlist checks whether incoming mail was sent by a subscriber to an email discussion list. If a mail was indeed sent by a subscriber to a configured list, tomlist distributes it to the other subscribers. If the incoming mail was sent to a valid list by a non-subscriber, tomlist forwards the mail to the configured "manager". If incoming mail is not addressed to a configured list, tomlist does nothing.
Configure email address from the same (sub)domain all to be forwarded. I point MX records to FastMail and configure FastMail to forward to firstname.lastname@example.org all mail sent for lists.thomaslevine.com.
Then I install tomlist on the SDF MetaArray. An example is provided. I could install it like so.
make rsync -avHSz pkg/tomlist-example/ email@example.com: