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https://play.google.com/store/apps/details?id=com.fyncom.rob...

Nano currency (nano.org) makes this possible and has some working implementations.

This is also what we do with bitmaelum[1]. Each message needs a proof of work for deliverance. Large (authentic) mailinglists can use an opt-in by a reader which allows the sender to bypass the pow restriction.

[1] https://github.com/bitmaelum

I've used this bot: https://twitter.com/lntxbot

takes microscopic amounts via Bitcoin's Lightning Network. Amazingly effective.

How is that better than any simple captcha?

I've had 5-6 different captcha bots, and they were routinely defeated.

I've used this bot for microtransactions: https://twitter.com/lntxbot

Captcha requires manual intervention, and potentially has the problem of being easy for bots but difficult for humans. A captcha for each email on even a small mailing list would be pretty horrible.