mx.jon.cz has no CNAME records.
Question and response
Question
dig mx.jon.cz. NS
Answer
The response has no answer records.
Authority
jon.cz. 3582 SOA ns.dns4.cz. hostmaster.active24.cz. 2026012103 10800 1800 604800 3600
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0