smtp-sec.interdns.co.uk has no CNAME records.
Question and response
Question
dig smtp-sec.interdns.co.uk. NS
Answer
The response has no answer records.
Authority
interdns.co.uk. 3600 SOA ns1.interdns.co.uk. admin.interdns.co.uk. 1410993357 28800 7200 604800 3600
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0