67.106.83.0.ptr.us.xo.net has no MX records.
Question and response
Question
dig 67.106.83.0.ptr.us.xo.net. MX
Answer
The response has no answer records.
Authority
xo.net. 900 SOA ns-twbg.verizonwireless.com. ipadmin.eng.xo.com. 2006052818 1800 600 604800 900
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0