Apex domains cannot have CNAME records.
Question and response
Question
dig proofhubmail.com. NS
Answer
proofhubmail.com. 4 NS ns-791.awsdns-34.net.
proofhubmail.com. 4 NS ns-488.awsdns-61.com.
proofhubmail.com. 4 NS ns-1652.awsdns-14.co.uk.
proofhubmail.com. 4 NS ns-1436.awsdns-51.org.
Authority
The response has no authority records.
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0