mx.softlayer.com has no CNAME records.
Question and response
Question
dig mx.softlayer.com. NS
Answer
The response has no answer records.
Authority
softlayer.com. 623 SOA ns1.softlayer.com. root.softlayer.com. 2025102200 7200 600 1728000 900
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0