Apex domains cannot have CNAME records.
Question and response
Question
dig mailjet.com. NS
Answer
mailjet.com. 5 NS ns1.mljt.tech.
mailjet.com. 5 NS ns0.mljt.tech.
mailjet.com. 5 NS ns2.mljt.tech.
Authority
The response has no authority records.
Additional
ns0.mljt.tech. 5 A 104.199.73.221
ns1.mljt.tech. 5 A 35.238.164.182
ns2.mljt.tech. 5 A 212.47.237.20
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0