Apex domains cannot have CNAME records.
Question and response
Question
dig oxismtp.com. NS
Answer
oxismtp.com. 5 NS nssec.online.net.
oxismtp.com. 5 NS ns.oxismtp.com.
Authority
The response has no authority records.
Additional
ns.oxismtp.com. 5 A 212.129.60.10
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0