cass.centos-server.net has no MX records.
Question and response
Question
dig cass.centos-server.net. MX
Answer
The response has no answer records.
Authority
centos-server.net. 86400 SOA ns1.wolverinedns.com. monitor.ace-host.net. 2026010701 3600 1800 1209600 86400
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0