It sounds like you are saying "there are no standalone SMTP clients", after which we would all start listing software that speaks SMTP.
There is a lot of software that speaks the ActivityPub server-server protocol, but most client apps use the Mastodon client API or custom protocols, not ActivityPub client-server.
It sounds like you are saying "there are no standalone SMTP clients", after which we would all start listing software that speaks SMTP.