On federated sites, you have to think of domain names as part of the username. The account isn't just @NASA. It is @NASA@mastodon.social, for example. This isn'

On federated sites, you have to think of domain names as part of the username. The account isn't just @NASA. It is @NASA@mastodon.social, for example. This isn't even a new concept. NASA@gmail.com and NASA@outlook.com can co-exist. We just have to get used to it outside of Email.