Apex domains cannot have CNAME records.
Question and response
Question
dig mta.getcheckrecipient.com. NS
Answer
mta.getcheckrecipient.com. 1 NS ns-171.awsdns-21.com.
mta.getcheckrecipient.com. 1 NS ns-1861.awsdns-40.co.uk.
mta.getcheckrecipient.com. 1 NS ns-1303.awsdns-34.org.
mta.getcheckrecipient.com. 1 NS ns-972.awsdns-57.net.
Authority
The response has no authority records.
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0