Apex domains cannot have CNAME records.
Question and response
Question
dig discoursemail.com. NS
Answer
discoursemail.com. 4 NS ns-1674.awsdns-17.co.uk.
discoursemail.com. 4 NS ns-143.awsdns-17.com.
discoursemail.com. 4 NS ns-748.awsdns-29.net.
discoursemail.com. 4 NS ns-1338.awsdns-39.org.
Authority
The response has no authority records.
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0