mail.s433.sureserver.com has no MX records.
Question and response
Question
dig mail.s433.sureserver.com. MX
Answer
The response has no answer records.
Authority
sureserver.com. 2560 SOA sgate1.sureserver.com. hostmaster.sureserver.com. 1772123615 16384 2048 1048576 2560
Additional
The response has no additional records.