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#hashCash is a proof-of-work anti-spam system for email proposed in 1997. People wanting to send an email would need to brute-force calculate a value (nonce) and include it the header.
It never really took off in itself, but became a building block for #bitcoin
#PoW understandably has a Very Bad Reputation for ecological through bitcoin, however its use as anti-spam has advantages, and is used by the accessibility friendly CAPTCHA system, #mCaptcha https://mcaptcha.org/
@douginamug Can we get a proof-of-work system that doesn't rely on increasing energy consumption? Greylisting is also a kind of "proof of work": to get your messages delivered you need to prove you have put effort on a queue of retries.
https://en.wikipedia.org/wiki/Greylisting_(email)
#smtp #email