mx1.talktalkplc.com has no CNAME records.
Question and response
Question
dig mx1.talktalkplc.com. NS
Answer
The response has no answer records.
Authority
talktalkplc.com. 1187 SOA ds0.opaltelecom.net. noc.opaltelecom.net. 2025110500 1200 3600 1728000 1200
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0