mail.bbtel.com has no CNAME records.
Question and response
Question
dig mail.bbtel.com. NS
Answer
The response has no answer records.
Authority
bbtel.com. 3540 SOA ns1.bbtel.com. adminblue.bbtel.com. 520800125 1200 180 1209600 3600
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0