mx2.hc5751-2.iphmx.com has no CNAME records.
Question and response
Question
dig mx2.hc5751-2.iphmx.com. NS
Answer
The response has no answer records.
Authority
iphmx.com. 3600 SOA ns1-93.akam.net. stbu-hostmaster\@cisco.com. 1771858650 3600 900 2419200 3600
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0