mx.stable.cz has no CNAME records.
Question and response
Question
dig mx.stable.cz. NS
Answer
The response has no answer records.
Authority
stable.cz. 3572 SOA ns.stable.cz. hostmaster.stable.cz. 2026021701 32400 7200 604800 3600
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0