Apex domains cannot have CNAME records.
Question and response
Question
dig mx.medallia.com. NS
Answer
mx.medallia.com. 4 NS ns-979.awsdns-58.net.
mx.medallia.com. 4 NS ns-1245.awsdns-27.org.
mx.medallia.com. 4 NS ns-1583.awsdns-05.co.uk.
mx.medallia.com. 4 NS ns-314.awsdns-39.com.
Authority
The response has no authority records.
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0