Hashcash is one of the many anti-spam techniques used today, which is essentially a proof-of-work system designed to limit not only email spam but also denial-of-service attacks. A similar proof-of-work methodology is being used in bitcoin for block generation.

With an average of 150 billion e-mails sent out on a daily basis I cannot see how this could be deployed in the blockchain, which would then have to be adjusted to facilitate 1,7 million TPS while now it only serves 7 TPS (artificial limit).

Also, bitcoin is not really made for such small payments which would result in tons of useless and unspendable dust outputs.