@username@domain. In practical terms, @user@example.com is not the same as @user@example.org. If the domain is not included, Mastodon will try to find a local user named @username. However, in order to deliver to someone over ActivityPub, the @username@domain mention is not enough – mentions must be translated to an HTTPS URI first, so that the remote actor’s inbox and outbox can be found.', 'activitypub' ), $allow_html ); ?>