Apex domains cannot have CNAME records.
Question and response
Question
dig paubox.com. NS
Answer
paubox.com. 5 NS ns-1972.awsdns-54.co.uk.
paubox.com. 5 NS ns-790.awsdns-34.net.
paubox.com. 5 NS ns-222.awsdns-27.com.
paubox.com. 5 NS ns-1114.awsdns-11.org.
Authority
The response has no authority records.
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0