<a href="http://www.apps.ietf.org/rfc/rfc1123.html#sec-6.1.3.5" rel="nofollow">http://www.apps.ietf.org/rfc/rfc1123.html#sec-6.1.3.5</a><p>Looks like underscores are valid in /domain names/ but not in /host names/.<p>You'll see underscores used many times for SPF records and in Microsoft's Active Directory.<p>Also, when you use dig I'm pretty sure it manually recurses the name server hierarchy while an application will just connect to your local name server which might have its own policies.