mail.sureserver.com has no CNAME records.
Question and response
Question
dig mail.sureserver.com. NS
Answer
The response has no answer records.
Authority
sureserver.com. 2505 SOA sgate1.sureserver.com. hostmaster.sureserver.com. 1772123615 16384 2048 1048576 2560
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0