Apex domains cannot have CNAME records.
Question and response
Question
dig hostedmxserver.com. NS
Answer
hostedmxserver.com. 5 NS ns-879.awsdns-45.net.
hostedmxserver.com. 5 NS ns-1611.awsdns-09.co.uk.
hostedmxserver.com. 5 NS ns-270.awsdns-33.com.
hostedmxserver.com. 5 NS ns-1049.awsdns-03.org.
Authority
The response has no authority records.
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0