Apex domains cannot have CNAME records.
Question and response
Question
dig s433.smtp-spf.sureserver.com. NS
Answer
s433.smtp-spf.sureserver.com. 5 NS ns1.s433.sureserver.com.
s433.smtp-spf.sureserver.com. 5 NS ns2.s433.sureserver.com.
Authority
The response has no authority records.
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0