Apex domains cannot have CNAME records.
Question and response
Question
dig paubox.net. NS
Answer
paubox.net. 4 NS ns-1136.awsdns-14.org.
paubox.net. 4 NS ns-652.awsdns-17.net.
paubox.net. 4 NS ns-405.awsdns-50.com.
paubox.net. 4 NS ns-1638.awsdns-12.co.uk.
Authority
The response has no authority records.
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0