smtp1.bt.com has no CNAME records.
Question and response
Question
dig smtp1.bt.com. NS
Answer
The response has no answer records.
Authority
bt.com. 600 SOA EDDNS0.BT.COM. zzdnsr.bt.com. 2006031107 28800 7200 604800 600
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0