SMTP was already entrenched in business uses and thus will survive for some time. XMPP was used by some chat servers. I used XMPP/Jabber in a company but we

SMTP was already entrenched in business uses and thus will survive for some time. XMPP was used by some chat servers. I used XMPP/Jabber in a company but we could have easily switched to something else such as IRC or NNTP. Moving off SMTP would be a non starter for most businesses. If there were an alternative to SMTP I believe it would be NNTP given it is similar in concept and just introduces threaded messaging which SMTP clients try to mimic now. There are several potential rabbit hole discussions of why moving away from SMTP are highly unlikely to ever occur.