s433.smtp-spf.sureserver.com has no MX records.
Question and response
Question
dig s433.smtp-spf.sureserver.com. MX
Answer
The response has no answer records.
Authority
s433.smtp-spf.sureserver.com. 2560 SOA ns1.s433.sureserver.com. hostmaster.s433.smtp-spf.sureserver.com. 1772242653 16384 2048 1048576 2560
Additional
The response has no additional records.