mxcluster03.adacor.com has no CNAME records.
Question and response
Question
dig mxcluster03.adacor.com. NS
Answer
The response has no answer records.
Authority
adacor.com. 2313 SOA ns.europe.adacor.net. hostmaster.adacor.net. 2026021102 10000 1800 604800 3600
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0