On Tue, Oct 02, 2018 at 15:40:19 +0200,
Stop sending e-mail to public mailing lists using Gmail, then? Or are
you going to pay for affected people's bandwith? Mailing lists have push
delivery model, so it's not possible to filter-out only HTML e-mail
before it's already downloaded.
It is, but it has to be done server side. I think mailman supports dropping
html attachments which should work pretty well for multipart/alternative
messages. For html only messages you would either need to reject them or
rewrite them, both of which have issues.