mx2.cbsmail.cz has no CNAME records.
Question and response
Question
dig mx2.cbsmail.cz. NS
Answer
The response has no answer records.
Authority
cbsmail.cz. 3600 SOA ns1.cbsn.cz. root.cbsn.cz. 1712602765 3600 900 604800 86400
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0