Apex domains cannot have CNAME records.
Question and response
Question
dig zaf-ca.p10.spamhero.com. NS
Answer
zaf-ca.p10.spamhero.com. 5 NS ns.dnsbox.net.
zaf-ca.p10.spamhero.com. 5 NS ns3.dnsbox.net.
zaf-ca.p10.spamhero.com. 5 NS ns2.dnsbox.net.
Authority
The response has no authority records.
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0