@Wolf480pl @jerry @Aaron exectamente! But we have to be explicit about what we're talking about (incidentally, anyone who heard me talk in private about infosec and internet messaging knows just *how* explicit I get, but I digress).
The last two of your points is what I meant by "infra around it". That's the hard part. The network effect.
But we have a head start since SMTP is an open standard.