Last time I checked the RFC the bit before the '@' can actually be empty. And the root zone is just '.', but we generally can leave off the trailing dot in domain names. So I believe '@' on its own is a valid email address.

Many years ago I managed to get a cctld owner to point their tld MX records at my smtp server, modified postfix to allow empty usernames (even although the RFC allows them, postfix didn't), and successfully had someone send me email to '@tld', in order to win a bet. :) (And it was a 2-letter tld.)