Apex domains cannot have CNAME records.
Question and response
Question
dig messagelabs.net. NS
Answer
messagelabs.net. 5 NS ns-228.awsdns-28.com.
messagelabs.net. 5 NS ns-1640.awsdns-13.co.uk.
messagelabs.net. 5 NS ns-1398.awsdns-46.org.
messagelabs.net. 5 NS ns-569.awsdns-07.net.
Authority
The response has no authority records.
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0