mx6.coprosys.cz has no CNAME records.
Question and response
Question
dig mx6.coprosys.cz. NS
Answer
The response has no answer records.
Authority
coprosys.cz. 360 SOA nsa.coprosys.cz. hostmaster.coprosys.cz. 2012110100 10800 3600 2419200 360
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0