mx.propersupport.com has no CNAME records.
Question and response
Question
dig mx.propersupport.com. NS
Answer
The response has no answer records.
Authority
propersupport.com. 1463 SOA propersupport.com. root.propersupport.com. 1676297270 28800 7200 604800 86400
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0