Apex domains cannot have CNAME records.
Question and response
Question
dig mailfiltering.com. NS
Answer
mailfiltering.com. 5 NS ns1.softlayer.com.
mailfiltering.com. 5 NS ns2.softlayer.com.
Authority
The response has no authority records.
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0