mx.nic.cz has no CNAME records.
Question and response
Question
dig mx.nic.cz. NS
Answer
The response has no answer records.
Authority
nic.cz. 1800 SOA a.ns.nic.cz. hostmaster.nic.cz. 1772119832 14400 3600 1209600 7200
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0