Creating the relay is the hard part of any mail service because even SMTP aside, you need to set DNS entries on your FROM domain to authorise your SMTP relay's IP (otherwise spam filters will just block your email). And if it's regular commercial mailshots then you'd need to register with one of the trusted services (which isn't free) and have all sorts of controls like rate limiting in place - again so you don't just get rejected as spam.
Honestly, I would welcome a complete ground up re-implementation of email - SMTP, POP/IMAP and all. The "very simple protocol, that's matured for decades now" is really just a mess of different protocols - different authentication models, different encryption methods, etc and all operating without a standard format for error handling. Plus all of the additional bloat bolted on to handle the swell of abuse that the email "network" has had to endure. There's nothing simple about email any more.
However going back to the topic: the unfortunate thing about this REST API is it seeks to solve the least complicated part of the whole SMTP stack.
