Apex domains cannot have CNAME records.
Question and response
Question
dig rpost.net. NS
Answer
rpost.net. 5 NS ns-1636.awsdns-12.co.uk.
rpost.net. 5 NS ns-1330.awsdns-38.org.
rpost.net. 5 NS ns-498.awsdns-62.com.
rpost.net. 5 NS ns-763.awsdns-31.net.
Authority
The response has no authority records.
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0