[fanf2]

If you are planning a proof-of-work system for email, you need to address the issues in this paper: https://www.cl.cam.ac.uk/~rnc1/proofwork2.pdf

And since blockchains are public, you need to address anonymity and communications tracking (what the spooks call metadata): public keys are pseudonyms and there has been a lot of successful research on decloaking “anonymized” data.

[phkahler]

You just need a handshake where the receiver sets the level of work required. Now you can whitelist people. You can also provide modest levels of work for "more reputable" email addresses, and set the default high for everyone else.

Done.

[rmhsilva]

This is great, thanks for the link. Love how it was written in 2004.