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 [email protected] 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. [email protected] 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.