mxserver6.strikemail.co.uk has no CNAME records.
Question and response
Question
dig mxserver6.strikemail.co.uk. NS
Answer
The response has no answer records.
Authority
strikemail.co.uk. 3600 SOA ns1.katapult.io. dns-admin.katapult.io. 1771411847 28800 7200 604800 3600
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0