I would like to add belatedly that I'm glad you're interested in this and don't want to discourage you from researching this area; I just think you should read

I would like to add belatedly that I'm glad you're interested in this and don't want to discourage you from researching this area; I just think you should read the hashcash/rpow history including problems that people ran into, because -- like most things having to do with spam -- I think it turned out to be a much thornier issue than anyone expected. So I think we don't have a clear path to a solution based on this research at present, because of things like spammers buying what we would now call mining hardware in order to outcompete regular people using a desktop PC, or spammers buying time on botnets for the same reason. But, it's very worth thinking about whether there are approaches that don't have this problem. In a way, that question is parallel to the ongoing debate in cryptocurrency mining about whether there's a way to do a mining-based blockchain that will continue to be decentralized and involve regular people's devices somehow, and that won't even up giving an advantage to centralized miners who invest a lot of capital into server farms and custom hardware. I know one recent entrant in this area is Chia which uses a proof-of-storage mechanism; I don't know whether that actually succeeds in reversing economic pressures toward centralization, but it could be interesting to look into whether its structure and incentives could somehow be bootstrapped into a "apparently, someone probably dedicated some moderately expensive resources that are probably on an ordinary PC, for a moderate period of time, in order to send you this message". And again, without ultimately getting hijacked by botnets that can make third parties' PCs participate illicitly. :-(