Apex domains cannot have CNAME records.
Question and response
Question
dig eyepinsmtp.com. NS
Answer
eyepinsmtp.com. 5 NS nsb.eyepin.at.
eyepinsmtp.com. 5 NS nsa.eyepin.at.
Authority
The response has no authority records.
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0