mx.dka.mailcore.net has no CNAME records.
Question and response
Question
dig mx.dka.mailcore.net. NS
Answer
The response has no answer records.
Authority
mailcore.net. 417 SOA ns21.cloudns.net. postmaster.mailcore.com. 2026021806 7200 1800 1209600 3600
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0