Apex domains cannot have CNAME records.
Question and response
Question
dig mx.a.cloudfilter.net. NS
Answer
mx.a.cloudfilter.net. 3 NS ns-1217.awsdns-24.org.
mx.a.cloudfilter.net. 3 NS ns-259.awsdns-32.com.
mx.a.cloudfilter.net. 3 NS ns-913.awsdns-50.net.
mx.a.cloudfilter.net. 3 NS ns-1804.awsdns-33.co.uk.
Authority
The response has no authority records.
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0