Apex domains cannot have CNAME records.
Question and response
Question
dig mx.e.sparkpost.com. NS
Answer
mx.e.sparkpost.com. 5 NS ns-867.awsdns-44.net.
mx.e.sparkpost.com. 5 NS ns-1132.awsdns-13.org.
mx.e.sparkpost.com. 5 NS ns-2015.awsdns-59.co.uk.
mx.e.sparkpost.com. 5 NS ns-93.awsdns-11.com.
Authority
The response has no authority records.
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0