This use case is actually the reason we have bitcoin, tho it's a shame it took off as a protest to banks instead of getting implemented as a functional anti-spam tool. To this day spam is regularly in my inbox and I have to check my spam folder for legitamate messages.
There's nothing stopping this from happening. Build encryption into the email protocols and hide the decrypt key behind a block chain that costs even $0.01 of a fungible token to access.
the blockchain, as usual, is not necessary, the header is proof enough on that a penny worth of energy[0] was expended in generating the hash
[0] (or however much energy you want to demand before throwing the message in the spam folder - you just require whatever number of leading zeros in the hash ie crank up the difficulty, bitcoin's innovation was assigning ownership to these proofs-of-work via signatures and implementing a mechanism to update and broadcast that ownership)
https://en.wikipedia.org/wiki/Hashcash