ns2.stackdns.com has no MX records.
Question and response
Question
dig ns2.stackdns.com. MX
Answer
The response has no answer records.
Authority
stackdns.com. 300 SOA ns1.stackdns.com. hostmaster.stackdns.com. 1453467320 3600 900 1209600 300
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0