mx.nettendesign.com has no MX records.
Question and response
Question
dig mx.nettendesign.com. MX
Answer
The response has no answer records.
Authority
nettendesign.com. 3600 SOA dns.nettendesign.com. hostmaster.nettendesign.com. 1556508229 43200 3600 604800 3600
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0