Yes I do suppose that's true, but I don't think that makes it any less proprietary in nature. The same could be said of many mail providers such as Gmail, Outlook.com, etc. They have their own way of doing it that, externally, looks like mail. Internally it's something altogether different.

This is proven by sending an email from one Gmail account to another. It is all but instant. I swear once the email arrived before the Enter key had debounced! SMTP can't move things quite that fast.