Indeed but what's the purpose of that. The SMTP server knows the IP address anyway and can DNS lookup if needed. The one chatting with server can say hello followed by any arbitrary string

SMTP (1982) predates DNS (1985).

Reverse DNS was added to SMTP servers as an anti-spam feature about 15 years after HELO was established.

But to be honest, I'm not sure HELO was ever really useful. In the very early years, it might have been appropriate to reject messages where the MAIL-FROM ___domain did not match the HELO ___domain. Not for long though.

This is the crazy thing about email, it's ancient, 39 years old. I bet it'll still be around in 20 years from today.

Ah! That was the missing information. Thanks, now it makes sense

HELO is not really used that much anymore. It's EHLO now. And response to it includes capabilities of the server. So it serves negotiation of those at the beginning of the session, too.

It's better to be prompted for those, instead of just sending them unannounced, in case client just want to switch to TLS first via starttls.

> can DNS lookup if needed

Can it? Not every machine that sends mail is on the public Internet.