mail.core.calserver.eu has no CNAME records.
Question and response
Question
dig mail.core.calserver.eu. NS
Answer
The response has no answer records.
Authority
core.calserver.eu. 2836 SOA ns1.cal.pl. hostmaster.core.calserver.eu. 2023102400 3600 3600 1209600 86400
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0