mx1.bbtel.com has no CNAME records.
Question and response
Question
dig mx1.bbtel.com. NS
Answer
The response has no answer records.
Authority
bbtel.com. 2415 SOA ns1.bbtel.com. adminblue.bbtel.com. 520800125 1200 180 1209600 3600
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0