You say that as though email is not, in fact, currently an absolute minefield of authentication, peering, trust, spam-guarding, and black-holing policies. In f

You say that as though email is not, in fact, currently an absolute minefield of authentication, peering, trust, spam-guarding, and black-holing policies. In fact, email is more-or-less the most-used example of how a federated system deals with this: sloppily, one-at-a-time, generating a massive haves-and-have-nots Balkanization. I can set up my own self-hosted mail server, but without some very careful configuration and planning good luck getting Gmail to forward its output to Google's users.