ns4.gdnsec.com has no MX records.
Question and response
Question
dig ns4.gdnsec.com. MX
Answer
The response has no answer records.
Authority
gdnsec.com. 10800 SOA tin.gdnsec.com. gdnssupport.nexusguard.com. 2015071601 10800 120 604800 10800
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0