[sushibowl]

The "chain" element is completely unnecessary here, since emails do not need to be strictly ordered (as opposed to currency transactions, where order is crucial). So all you need is to consider each e-mail like a standalone block and apply a proof of work to it.

This is not new, by the way. Preventing spam was the original application for PoW way before the blockchain existed. First proposed in 1997 with https://en.wikipedia.org/wiki/Hashcash

The downside is that it can be tricky to correctly tune the difficulty factor for a wide variety of e-mail sending hardware with wildly varying computing power available.