_external.virginmedia.com has no CNAME records.
Question and response
Question
dig _external.virginmedia.com. NS
Answer
The response has no answer records.
Authority
virginmedia.com. 1800 SOA ns1.virginmedia.net. hostmaster.virginmedia.net. 2025091280 86400 7200 2419200 1800
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0