At this point, our email validity criteria: .+@.+\..{2,} That is, at least one character for the inbox, at least one character for the domain, at least two for the TLD (we assume that TLD-less domains are undeliverable by us). This ensures we don't allow 'a@a' or 'a@a.a', but do allow 'a@a.io'.
[EDIT: I see that you consider TLD-free e-mail addresses undeliverable; still!]