Apex domains cannot have CNAME records.
Question and response
Question
dig r3.rpost.net. NS
Answer
r3.rpost.net. 5 NS ns-1345.awsdns-40.org.
r3.rpost.net. 5 NS ns-685.awsdns-21.net.
r3.rpost.net. 5 NS ns-143.awsdns-17.com.
r3.rpost.net. 5 NS ns-1618.awsdns-10.co.uk.
Authority
The response has no authority records.
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0