smtp.mobileapproval.bigmachines.com has no MX records.
Question and response
Question
dig smtp.mobileapproval.bigmachines.com. MX
Answer
The response has no answer records.
Authority
mobileapproval.bigmachines.com. 300 SOA ns1.p201.dns.oraclecloud.net. hostmaster.mobileapproval.bigmachines.com. 2016060207 3600 600 604800 1800
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0