Apex domains cannot have CNAME records.
Question and response
Question
dig mailserver-od.com. NS
Answer
mailserver-od.com. 4 NS ns-884.awsdns-46.net.
mailserver-od.com. 4 NS ns-137.awsdns-17.com.
mailserver-od.com. 4 NS ns-1589.awsdns-06.co.uk.
mailserver-od.com. 4 NS ns-1485.awsdns-57.org.
Authority
The response has no authority records.
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0