mx.stable.sk has no CNAME records.
Question and response
Question
dig mx.stable.sk. NS
Answer
The response has no answer records.
Authority
stable.sk. 4253 SOA ns.stable.cz. hostmaster.stable.cz. 2026012504 28800 7200 604800 10800
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0