_spf.interdns.co.uk has no MX records.
Question and response
Question
dig _spf.interdns.co.uk. MX
Answer
The response has no answer records.
Authority
interdns.co.uk. 3600 SOA ns1.interdns.co.uk. admin.interdns.co.uk. 1410993336 28800 7200 604800 3600
Additional
. 0 OPT ; payload 4000, xrcode 0, version 0, flags 0