Apex domains cannot have CNAME records.
Question and response
Question
dig mail.e.sparkpost.com. NS
Answer
mail.e.sparkpost.com. 5 NS ns-699.awsdns-23.net.
mail.e.sparkpost.com. 5 NS ns-122.awsdns-15.com.
mail.e.sparkpost.com. 5 NS ns-1773.awsdns-29.co.uk.
mail.e.sparkpost.com. 5 NS ns-1031.awsdns-00.org.
Authority
The response has no authority records.
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0