mx1.nominet.org.uk has no CNAME records.
Question and response
Question
dig mx1.nominet.org.uk. NS
Answer
The response has no answer records.
Authority
nominet.org.uk. 300 SOA dns1.nominetdns.uk. hostmaster.nominet.org.uk. 1716211162 7200 900 2419200 3600
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0