Apex domains cannot have CNAME records.
Question and response
Question
dig outsmtp.com. NS
Answer
outsmtp.com. 5 NS ns4.dnssunucu.com.
outsmtp.com. 5 NS ns1.dnssunucu.com.
outsmtp.com. 5 NS ns2.dnssunucu.com.
outsmtp.com. 5 NS ns3.dnssunucu.com.
Authority
The response has no authority records.
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0