[dsr_]

... what does that mean?

It sounds like you are saying "there are no standalone SMTP clients", after which we would all start listing software that speaks SMTP.

[remram]

So point out the clients that speak ActivityPub?

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.

[esjeon]

SMTP is strictly a server-to-server protocol. POP3 and IMAP are the protocols for accessing mailboxes remotely.

[cdot2]

You can send mail via smtp connection from client to server

[esjeon]

Woopsie, totally forgot about that part. It seems like I got too used to webmails.