ground.corbina.net has no MX records.
Question and response
Question
dig ground.corbina.net. MX
Answer
The response has no answer records.
Authority
corbina.net. 3600 SOA dns1.corbina.net. hostmaster.corbina.net. 1772171101 3600 300 1209600 3600
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0