Apex domains cannot have CNAME records.
Question and response
Question
dig smtp39.com. NS
Answer
smtp39.com. 5 NS ns58.domaincontrol.com.
smtp39.com. 5 NS ns57.domaincontrol.com.
Authority
The response has no authority records.
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0