in-addr.zen.co.uk has no MX records.
Question and response
Question
dig in-addr.zen.co.uk. MX
Answer
The response has no answer records.
Authority
in-addr.zen.co.uk. 86400 SOA unamuno.zen.co.uk. dnsmaster.zen.co.uk. 4 28800 7200 1209600 86400
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0