mailrelay01.ntl.com has no CNAME records.
Question and response
Question
dig mailrelay01.ntl.com. NS
Answer
The response has no answer records.
Authority
ntl.com. 1800 SOA ns1.virginmedia.net. hostmaster.virginmedia.net. 2021071238 86400 7200 1209600 1800
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0