We have Lemmy.world, Lemmy.ml, beehaw.org, I think even yiffit.net. If I log in using wefwef it asks me where I want to login. And when I make a new account using another instance, it says ”… Lemmy is federated, so you can interact with everything on lemmy.ml even if you’re registered on a different instance.“ So if I’m registered on lemmy.world, how can I interact with lemmy.ml?
The analogy isn’t meant to reflect who receives the message, but how communication interacts.
Each lemmy/federated instance uses a prefix and a suffix to create a name. So for example, I’m Melpomene@kbin.social here, where Melpomene is me and kbin.social is where I reside. This is similar to email because it works similarly… I might be Melpomene on Gmail, but my actual user name reflects both me (Melpomene) and the email instance I call home (gmail.com.)
If you want a specific analogy to messaging, though… when you send a message to a lemmy community, you’re basically responding to a distribution list. nostupidquestions@lemmy.world is akin a distribution list, the distribution list is (technically) visible to anyone on the instance, so anyone can see the message. If you’re on a federated instance that allows private messages, though, sending to an individual user would still function like email in that only the person messaged would see it.
Edit: The comparison is not exact, granted. It’s meant to de-mystify the prefix/suffix format of federation, not to offer a one to one comparison to email.