Your recommendation is almost as wrong as the regexp in the article, as both will reject the perfectly valid postmaster@ai address (check it out, "dig -t mx ai"

Your recommendation is almost as wrong as the regexp in the article, as both will reject the perfectly valid postmaster@ai address (check it out, "dig -t mx ai" will return a result, so that address must exist). The only thing you can be sure about in a non-local email address is that it contains at least one @ sign. More are allowed (for source routing), but hopefully no server is still configured to honor that, so you might get away with requiring exactly one @. Everything else is evil. Oh, and don't forget to make sure that one component in your spam^W email processing chain correctly encodes unicode charaters in the domain part into punycode.