Apex domains cannot have CNAME records.
Question and response
Question
dig smtp-od.com. NS
Answer
smtp-od.com. 5 NS ns-947.awsdns-54.net.
smtp-od.com. 5 NS ns-146.awsdns-18.com.
smtp-od.com. 5 NS ns-1547.awsdns-01.co.uk.
smtp-od.com. 5 NS ns-1357.awsdns-41.org.
Authority
The response has no authority records.
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0