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Some type of proof‐of‐work for email addresses that have not been previously white-listed. Similar to what BitMessage does[1].

Sending emails to new/random contacts shouldn't be that easy anyway.

[1] https://bitmessage.org/bitmessage.pdf

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Encrypting email to a single recipient is (a weak) proof of work.

There's a big difference between sending 10k copies of a plaintext email, and retrieving a public key and encrypting to 10k recipients - not to mention that filtering out all non signed/not-signed-by-trusted-key should be a decent start for a whitelist/greylist.

I'd be curious if anyone ever got gpg-encrypted spam?

Well, I doubt anyone gets GPG encrypted spam, because only a small number of people actually know how to decrypt GPG.

Another approach would be to have to pay the equivalent of USD 5-10 cents in some email crypto currency in order to allow an unknown sender to put their email on my inbox. If I reply or add the sender to my whitelist the fee is automatically refunded to the original sender and future messages between both parties would won't require a transaction fee any longer.

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