flat.outboundspamprotection.com has no MX records.
Question and response
Question
dig flat.outboundspamprotection.com. MX
Answer
The response has no answer records.
Authority
outboundspamprotection.com. 1800 SOA josh.ns.cloudflare.com. dns.cloudflare.com. 2397191889 10000 2400 604800 1800
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0