Apex domains cannot have CNAME records.
Question and response
Question
dig mail.vadesecure.com. NS
Answer
mail.vadesecure.com. 5 NS ns-1982.awsdns-55.co.uk.
mail.vadesecure.com. 5 NS ns-669.awsdns-19.net.
mail.vadesecure.com. 5 NS ns-233.awsdns-29.com.
mail.vadesecure.com. 5 NS ns-1125.awsdns-12.org.
Authority
The response has no authority records.
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0