Comment by uiri

SMTP is to Postfix as ActivityPub is to Mastodon. If you're not technical enough to care about the distinction between protocol and implementation (as most users are not), then the distinction is largely academic. You can register on a server running Mastodon and communicate with anyone on any other server running Mastodon or with anyone on any server running any of the other ActivityPub implementations (Pleroma, Misskey, etc).