Apex domains cannot have CNAME records.
Question and response
Question
dig mailhostingservice.com. NS
Answer
mailhostingservice.com. 5 NS ns-574.awsdns-07.net.
mailhostingservice.com. 5 NS ns-326.awsdns-40.com.
mailhostingservice.com. 5 NS ns-1718.awsdns-22.co.uk.
mailhostingservice.com. 5 NS ns-1506.awsdns-60.org.
Authority
The response has no authority records.
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0