mail.dc3.cz has no CNAME records.
Question and response
Question
dig mail.dc3.cz. NS
Answer
The response has no answer records.
Authority
dc3.cz. 600 SOA ns1.tele3.cz. root.tele3.cz. 1772606448 28800 7200 1209600 43200
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0