coremail01-old.mail.zen.net.uk has no MX records.
Question and response
Question
dig coremail01-old.mail.zen.net.uk. MX
Answer
The response has no answer records.
Authority
mail.zen.net.uk. 3600 SOA ns0.zen.co.uk. dnsmaster.zen.co.uk. 861 1800 120 604800 3600
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0