co.uk has no MX records.
Question and response
Question
dig co.uk. MX
Answer
The response has no answer records.
Authority
co.uk. 10800 SOA dns1.nic.uk. hostmaster.nominet.org.uk. 1771820145 900 300 2419200 10800
Additional
. 0 OPT ; payload 4096, xrcode 0, version 0, flags 0