mail.master.cz has no CNAME records.
Question and response
Question
dig mail.master.cz. NS
Answer
The response has no answer records.
Authority
master.cz. 300 SOA ns1.master.cz. noc.master.cz. 2026030300 900 900 1800 300
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0