Apex domains cannot have CNAME records.
Question and response
Question
dig mail03-userengage.com. NS
Answer
mail03-userengage.com. 5 NS ns1.mail03-userengage.com.
mail03-userengage.com. 5 NS ns2.mail03-userengage.com.
Authority
The response has no authority records.
Additional
ns1.mail03-userengage.com. 5 A 91.230.36.48
ns2.mail03-userengage.com. 5 A 18.195.124.99
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0