mx.mad.interhost.com has no CNAME records.
Question and response
Question
dig mx.mad.interhost.com. NS
Answer
The response has no answer records.
Authority
mad.interhost.com. 2759 SOA ns.interhost.com. hostmaster.mad.interhost.com. 2026022246 7200 1800 1209600 10799
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0