Also, RFC 5321 explicitly mentions the possibility of email addresses where the ___domain consists of just a TLD: http://tools.ietf.org/html/rfc5321#section-2.3.5
Non-authoritative answer: Name: ac Address: 193.223.78.210
then again my upstream provider defaults to opendns.com so http://ac in the browser does not work.
Also, RFC 5321 explicitly mentions the possibility of email addresses where the ___domain consists of just a TLD: http://tools.ietf.org/html/rfc5321#section-2.3.5