smtp-out.inf.bulletproof.net has no MX records.
Question and response
Question
dig smtp-out.inf.bulletproof.net. MX
Answer
The response has no answer records.
Authority
inf.bulletproof.net. 600 SOA ns0.bulletproof.net.au. hostmaster.bulletproof.net.au. 1772611355 10800 3600 604800 600
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0