Such a product would need to have a ToS that states it can only be used for low volume personal and business correspondence, and have per-account rate limiting to enforce that. Doing it this way is necessary to avoid having mail treated as bulk.
I've tried relaying through mailgun to get mail to outlook.com, it does not work.
Jk, I just thought it was funny that we pay for services which were always seen as a free alternative to paper mail and stamps were a thing of the past.
On top of that, the costs of electricity are not the same everywhere (they are very expensive in The Netherlands, for example, than in the USA, and China is very cheap). Nor is it widely stable available everywhere (examples include India and Africa). Proof of Work is basically a waste of energy as well.
Maybe it's just been so long I'm whitelisted?
That's basically Sendgrid or Mailgun or even Fastmail itself. They can all relay SMTP for arbitrary domains.