185-151-30-0.ptr4.stackcp.net has no MX records.
Question and response
Question
dig 185-151-30-0.ptr4.stackcp.net. MX
Answer
The response has no answer records.
Authority
stackcp.net. 300 SOA ns1.stackdns.com. hostmaster.stackdns.com. 1465287633 1800 900 1209600 300
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0