cas.corbina.net has no MX records.
Question and response
Question
dig cas.corbina.net. MX
Answer
The response has no answer records.
Authority
corbina.net. 3600 SOA dns1.corbina.net. hostmaster.corbina.net. 1772085901 3600 300 1209600 3600
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0