192.173.207.in-addr.arpa has no MX records.
Question and response
Question
dig 192.173.207.in-addr.arpa. MX
Answer
The response has no answer records.
Authority
192.173.207.in-addr.arpa. 7200 SOA ns.integraonline.com. hostmaster.integraonline.com. 2008081201 2800 7200 604800 86400
Additional
. 0 OPT ; payload 4096, xrcode 0, version 0, flags 0