>I think this is what federation looks like when it works. Yeah, everyone uses Gmail, but it's also ok to use another service. It still works and delivers a pretty similar experience. Your messages make it out of the Gmail garden.

That's not true "federation" the way the OP meant. What's happened with email is that it's turned into an oligopoly: you can have email from Gmail, Apple, Yahoo (if you're elderly), or Microsoft, and that's it, unless you have a corporate address for work. If you want to run your own personal email server, forget it: you'll be considered a spammer and your emails will go into the bit-bucket.