mx151.softlayer.com has no CNAME records.
Question and response
Question
dig mx151.softlayer.com. NS
Answer
The response has no answer records.
Authority
softlayer.com. 731 SOA ns1.softlayer.com. root.softlayer.com. 2025102200 7200 600 1728000 900
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0