ukcore.bt.net has no MX records.
Question and response
Question
dig ukcore.bt.net. MX
Answer
The response has no answer records.
Authority
ukcore.bt.net. 600 SOA ns0.bt.net. hostmaster.bt.net. 2016090141 28800 7200 604800 600
Additional
. 0 OPT ; payload 4096, xrcode 0, version 0, flags 0