mx.colorbusline.com has no CNAME records.
Question and response
Question
dig mx.colorbusline.com. NS
Answer
The response has no answer records.
Authority
colorbusline.com. 3309 SOA dns.colorbusline.com. hostmaster.colorbusline.com. 1896443729 43200 3600 604800 3600
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0