mx.inetadmin.cz has no CNAME records.
Question and response
Question
dig mx.inetadmin.cz. NS
Answer
The response has no answer records.
Authority
inetadmin.cz. 506 SOA ns.inetadmin.eu. root.inetadmin.eu. 1771459200 86400 900 1209600 900
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0