My idea for blockchain is to encrypt emails with the password stored on the chain. The mail receive client will reject messages that are unencrypted, and in order to access the key, the sender will need to send a micropayment, say $0.05, of a for-real, redeemable to fiat, coin. As the funds are claimed by the recipient, the key is revealed and the message is decrypted. After some configurable timeout after reading the message, the micropayment goes back to the sender. Unless the recipient clicks "this is spam". In that case the money is kept. This could potentially end spam as we know it, because suddenly if you want to send a million messages, now you need to put up $50,000.

The challenge here would be keeping the transaction costs as low as possible and using a stable coin. It would cost everyone a couple bucks to send messages, which is a nuisance, and some people might be tempted to steal nickels for various reasons. The crypto would be there to prove there is no way the recipient can possibly receive your message without putting up cash. Perhaps some very popular people could set their threshold higher to reduce noise. But even $0.01 would be highly effective.