mail.nic.gdn has no MX records.
Question and response
Question
dig mail.nic.gdn. MX
Answer
The response has no answer records.
Authority
nic.gdn. 900 SOA ns1.nic.gdn. postmaster.nic.gdn. 1501498516 21600 1800 2419200 900
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0