nsd.nic.uk has no MX records.
Question and response
Question
dig nsd.nic.uk. MX
Answer
The response has no answer records.
Authority
nic.uk. 172800 SOA dns1.nic.uk. hostmaster.nic.uk. 1768318877 7200 300 2419200 172800
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0