Apex domains cannot have CNAME records.
Question and response
Question
dig messagelabs.com. NS
Answer
messagelabs.com. 5 NS ns-1714.awsdns-22.co.uk.
messagelabs.com. 5 NS ns-453.awsdns-56.com.
messagelabs.com. 5 NS ns-520.awsdns-01.net.
messagelabs.com. 5 NS ns-1296.awsdns-34.org.
Authority
The response has no authority records.
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0