Hashcash was proposed for email spam protection, and Microsoft was trying this in the early 00s with the Penny Black project.
The problem with proof of work is that it’s hard to adjust difficulty without a feedback mechanism, Bitcoin solves this with 2016 block epochs.
Email and SMTP don’t have such mechanism, how do I know that spam farm just bought 500 new mail asic miners? does every mail front end now need to buy miners? Grandma using Eudora? If it’s CPU only, how do we agree to a given mining algo. SMTP is merely a protocol, it is not consensus. There is no consensus in mail beyond the very basics in handling — there is no lock step mechanism like Satoshi clients.
Most email today is sent from a handful of providers who can centrally filter and control spam.