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?
But if you both subscribe to the same mailing list you will both receive the same message.
I’m confident that “mailing lists” are not a thing most people will know about or understand today, and for a lot of people “email” and “gmail” are completely synonymous terms. A lot of people will have never interacted with an @yahoo account, I wasn’t aware they were even still in business.