_spf.zen.co.uk has no CNAME records.
Question and response
Question
dig _spf.zen.co.uk. NS
Answer
The response has no answer records.
Authority
zen.co.uk. 10800 SOA ns0.zen.co.uk. dnsmaster.zen.co.uk. 2026021951 2400 900 604800 86400
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0