Messages should be sent from client to client. The server layer can be skipped. But the client could of course be a cloud based email solution. Messages would n

Messages should be sent from client to client. The server layer can be skipped. But the client could of course be a cloud based email solution. Messages would need double opt in, which would be baked into the protocol. Id's would be a random hash. Personal messages could be trust based eg. "Do i know this person" and corporates could use certificates. Once a user has opted in to receive messages from a id/hash the receiver would generate a challenge at the start of each session, either using a shared secret or public key crypto (also baked into the protocol). The protocol should have a standard for sending simple text. But then allow extension protocols, for example implementing voice/video. With capabilities eg. Client tells the other client what formats it accepts.