Email is federated at a lower layer. At the UX layer, it is not federated. Fully qualified email addresses are like usernames. If you ask a random email user, they would think it works like this. If you ask a random Mastodon user, they know that they have to join a relatively isolated silo. Also most uses of email are peer-to-peer. In the case of two people emailing each other, the number of peers is 2. This is not directly comparable to public social networks.
Email was once the main way social networking was done via mailing lists. This was kind of federated, because you had to ask the listserv to add you to the list. The federated aspects of email are what caused it to die and get replaced by forums which in turn got replaced by Reddit and Twitter.
(Notice the trend? Mailing lists -> forums -> Reddit. From federated to centralized. Network effects.)