Apex domains cannot have CNAME records.
Question and response
Question
dig mailkit.com. NS
Answer
mailkit.com. 4 NS ns5.mailkit.com.
mailkit.com. 4 NS ns4.mailkit.com.
mailkit.com. 4 NS ns2.mailkit.com.
mailkit.com. 4 NS ns3.mailkit.com.
mailkit.com. 4 NS ns1.mailkit.com.
Authority
The response has no authority records.
Additional
ns1.mailkit.com. 4 A 217.11.236.82
ns2.mailkit.com. 4 A 185.136.200.18
ns3.mailkit.com. 4 A 35.245.60.94
ns4.mailkit.com. 4 A 128.199.136.226
ns5.mailkit.com. 4 A 103.125.216.244
ns4.mailkit.com. 4 AAAA 2400:6180:0:d0:0:0:d87:9001
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0