mail3.zcom.cz has no CNAME records.
Question and response
Question
dig mail3.zcom.cz. NS
Answer
The response has no answer records.
Authority
zcom.cz. 1800 SOA ns1.zcom.cz. hostmaster.zcom.cz. 1772788468 10800 3600 604800 1800
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0