Just come across this. Haven’t dug too deep but sounds like an interesting opportunity. I think it can only succeed if it is fully compliant with current systems. Seems to only be mobile clients. No desktop? Are big tech going to permit this to grow? It would shut down a data mining opportunity …
Its not difficult to self host email. Ispconfig will do all the work for you locally, then you just need to set up your DNS records. The difficult part is staying on top of patching and block lists.