Apex domains cannot have CNAME records.
Question and response
Question
dig mail.workfront.com. NS
Answer
mail.workfront.com. 5 NS ns-340.awsdns-42.com.
mail.workfront.com. 5 NS ns-2037.awsdns-62.co.uk.
mail.workfront.com. 5 NS ns-1275.awsdns-31.org.
mail.workfront.com. 5 NS ns-1007.awsdns-61.net.
Authority
The response has no authority records.
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0