Apex domains cannot have CNAME records.
Question and response
Question
dig sendpost.email. NS
Answer
sendpost.email. 5 NS ns-881.awsdns-46.net.
sendpost.email. 5 NS ns-1471.awsdns-55.org.
sendpost.email. 5 NS ns-1814.awsdns-34.co.uk.
sendpost.email. 5 NS ns-15.awsdns-01.com.
Authority
The response has no authority records.
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0