Apex domains cannot have CNAME records.
Question and response
Question
dig ersmtp.com. NS
Answer
ersmtp.com. 5 NS ns3.erado.net.
ersmtp.com. 5 NS ns1.erado.net.
ersmtp.com. 5 NS ns2.erado.net.
Authority
The response has no authority records.
Additional
ns1.erado.net. 5 A 207.195.163.37
ns2.erado.net. 5 A 63.71.10.37
ns3.erado.net. 5 A 199.30.232.37
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0