ns3.nudns.com has no MX records.
Question and response
Question
dig ns3.nudns.com. MX
Answer
The response has no answer records.
Authority
nudns.com. 3600 SOA ns1.nudns.com. admin.nudns.com. 2019102518 1200 900 604800 60
Additional
. 0 OPT ; payload 4096, xrcode 0, version 0, flags 0