Hi Peter,
I have a solution in mind: put the first mail from a new sender under review and then once accepted allow subsequent mails to pass through. This would hopefully thwart most bots, and wouldn't be as much of a burden as reviewing every mail.
You beat me to it, but I was going to suggest this too. It's what we have been using for FSCI (Free Software Community of India) mailing lists and has been working out quite well. A spam filter in front of that helps reduce the noise further, of course. -- *S. Badri* *Web:* badrihippo.thekambattu.rocks <https://badrihippo.thekambattu.rocks> *Chat:* badrihippo@disroot.org <https://invite.joinjabber.org/#badrihippo@disroot.org> *Social:* @badrihippo@fosstodon.org <https://fosstodon.org/@badrihippo>