Alternate proposals have failed and will always fail due to the ingrained nature of email. Nobody is going to use a new email system that allows communication to zero or very few recipients, and the system can never bootstrap. And that's that, no alternative to email will succeed if its only claim to fame is that is "email done right", that's a much too weak feature to break SMTP's decades of first mover advantage.
To break this clinch, you need to either:
1. deliver a communication product that has much superior features, and incidentally does mail very very well; people will start to use it for the other features, play with its email subset functionally, and in many, many, many years, the email will wither and die. Up to now, despite many claims to the contrary, no one managed to do that. The average lifespan of instant messaging protocols is around two decades (anyone on ICQ or Yahoo anymore?), while email is going strong into its 50s.
2. backport those advanced features into SMTP so that they are interoperable with the existing infrastructure AND, at the same time, deliver a tangible benefit to the people using those features, AND deliver those benefits today, not into some distant future where most people start to use it (PGP, where are you). This is a very hard set of conditions and very few technologies have managed to fulfill them, as a consequence email has evolved very slowly.
So if you don't want to have the old "canned letter response making fun of your antispam system" rubbed in your face, you better start working on number 1. or number 2.