Apex domains cannot have CNAME records.
Question and response
Question
dig premiumsmtp.dnshostservices.com. NS
Answer
premiumsmtp.dnshostservices.com. 5 NS ns1.dnshostservices.com.
premiumsmtp.dnshostservices.com. 5 NS ns4.dnshostservices.com.
premiumsmtp.dnshostservices.com. 5 NS ns3.dnshostservices.com.
premiumsmtp.dnshostservices.com. 5 NS ns2.dnshostservices.com.
Authority
The response has no authority records.
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0