fbsv.net has no MX records.
Question and response
Question
dig fbsv.net. MX
Answer
The response has no answer records.
Authority
fbsv.net. 3600 SOA a.ns.facebook.com. dns.facebook.com. 4207849484 14400 1800 604800 3600
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0