Apex domains cannot have CNAME records.
Question and response
Question
dig inbound-smtp.ca-central-1.amazonaws.com. NS
Answer
inbound-smtp.ca-central-1.amazonaws.com. 4 NS ns-216.awsdns-27.com.
inbound-smtp.ca-central-1.amazonaws.com. 4 NS ns-1619.awsdns-10.co.uk.
inbound-smtp.ca-central-1.amazonaws.com. 4 NS ns-1111.awsdns-10.org.
inbound-smtp.ca-central-1.amazonaws.com. 4 NS ns-847.awsdns-41.net.
Authority
The response has no authority records.
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0