I've known people back in the 90s with user@tld domains, and they were definitely sending and receiving mail. So, even if it wasn't spec-compliant it certainly got through all the early mail relays.
I mean.. you can use emoji domains right now. They work most places for email. The part I found didn't work so well is emoji usernames on emoji domains. That has poor deliverability.
ICANN, by the way, heavily discourages such domain names, even though it can't actually prohibit them: yes, RFC 5892 explicitly prohibits emoji code points in internationalized domain names but so what? If registrars allow (and many acually do allow) registration of such names that only means that they violate some RFC and they already violate quite a lot of them. Who cares! Just pay the money and we will delegate you whatever names you want.
I mean.. you can use emoji domains right now. They work most places for email. The part I found didn't work so well is emoji usernames on emoji domains. That has poor deliverability.