Apex domains cannot have CNAME records.
Question and response
Question
dig s1138.smtp-spf.sureserver.com. NS
Answer
s1138.smtp-spf.sureserver.com. 4 NS ns1.s1138.sureserver.com.
s1138.smtp-spf.sureserver.com. 4 NS ns2.s1138.sureserver.com.
Authority
The response has no authority records.
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0