The problem is, that email isn't really decentralised/federated anymore.
It's impossible to day to setup your own mailserver and have the email accepted by the major email providers (where most of the people are)!
Checkout this article: https://cfenollosa.com/blog/after-self-hosting-my-email-for-twenty-three-years-i-have-thrown-in-the-towel-the-oligopoly-has-won.html
It's sad, but while email is decentralised in it's core protocol, it's execution has become too centralised today. Fortunately, HTML and the web standards are better as we at least still have Firefox (a non-chromium BLINK engine based browser) today. But even there, chrome takes up too much of the market share.