a.ns.facebook.com has no MX records.
Question and response
Question
dig a.ns.facebook.com. MX
Answer
The response has no answer records.
Authority
facebook.com. 3600 SOA a.ns.facebook.com. dns.facebook.com. 4207849484 14400 1800 604800 300
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0