smtpb1.bt.com has no CNAME records.
Question and response
Question
dig smtpb1.bt.com. NS
Answer
The response has no answer records.
Authority
bt.com. 600 SOA EDDNS0.BT.COM. zzdnsr.bt.com. 2006031109 28800 7200 604800 600
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0