[wcoenen]

A TLD can have an A record. For example, http://ac/ leads to a webpage.

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

[nextstep]

http://ac/ didn't work for me.

[d503]

It doesn't work for me either, but http://ac./ does.

[lifeisstillgood]

nslookup

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.