#ActivityPub has a lot of the same characteristics as email. But in email, the duties of each part of the process are split up, and you can mix-and-match software. Fediverse software is more tightly coupled than that.
SMTP is like the ActivityPub server-to-server protocol. Like in email with sendmail/exim/postfix/etc, you could have a program that does just this.
IMAP is like the server-to-client protocol. Like in email, with courier/dovecot/etc, you could have a program that does just this.
