For personal use email is rarely self-hosted, but corporations, governments, and organisations often run their own email infrastructure. That effectively makes email a decentralized federated system.
You might argue how a lot of email is either send to or send from Google, Apple, or Microsoft services, and that is thus somewhat centralized, but isn't that stretching the definition?
You might argue how a lot of email is either send to or send from Google, Apple, or Microsoft services, and that is thus somewhat centralized, but isn't that stretching the definition?