Apex domains cannot have CNAME records.
Question and response
Question
dig smtpio.com. NS
Answer
smtpio.com. 5 NS isaac.ns.cloudflare.com.
smtpio.com. 5 NS laura.ns.cloudflare.com.
Authority
The response has no authority records.
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0