Fediverse
For example, Mastodon gGmbH has the fediverse instance “mastodon.social”, and Gmail has the email address “gmail.com”. A person on “mastodon.social” has the fediverse identity “@ username @mastodon.social”, and a person on “gmail.com” has the email address “ username @gmail.com”. But a “gmail.com” user can send an email to someone who uses a different email provider, like “hotmail.com”. Similarly, a “mastodon.social” user can comment on a post by someone who uses a different fediverse instance, like “mozilla.social”.