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.)
The Fediverse is federated on exactly the same layer as email. You have a username of the form user@domain,this is the same in both Mastodon and email.
You can send messages to anyone regardless of the instance they're on. This is also exactly the same on both Mastodon and email.
Precisely how are these different in any relevant way?
The fediverse has conversation streams that are mostly in public indexes, which other people can observe and interact with. In addition to these conversations comprised of statuses representing activities, it is also possible to follow the authors providing said content so that their latest posts show up in your stream, regardless of whether those posts pertained to that conversation or not.
Email, conversely, often is a limited-scope conversation that can only be observed or interacted with between the people participating in said message. Of course, this changes slightly with the use-case of mailing lists, which often provide a public archive of prior messages. But email content is generally not accessible from the web in the same manner that a status is.
Sure, but when I said "in a relevant way" I was talking about differences that are relevant in the context of the conversation.
There was nothing wrong in your description of the differences between the Fediverse and email, but none of those differences explain the previous poster's assertion that the protocols work at fundamentally different levels.
They don't, though[1]. It's often the lowest-common-denominator because everyone has it. But if a more streamlined alternative, like Slack, is available then it will be preferred over email.
A quick look at websites talking about email trends finds that the only people who are bullish on email are marketers. And that's why everyone else hates it: it's predominantly spam. So if you want to compare ActivityPub to email you'd better have a strong argument for why it won't just become another vehicle for spam if it ever becomes widespread.
Network effects can be achieved with or without federation. They aren't mutually exclusive.