ns31.constellix.com has no MX records.
Question and response
Question
dig ns31.constellix.com. MX
Answer
The response has no answer records.
Authority
constellix.com. 180 SOA ns11.constellix.com. dns.constellix.com. 2015010701 43200 3600 1209600 180
Additional
. 0 OPT ; payload 1280, xrcode 0, version 0, flags 0