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.

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