mx2.securitynet.cz has no CNAME records.
Question and response
Question
dig mx2.securitynet.cz. NS
Answer
The response has no answer records.
Authority
securitynet.cz. 714 SOA ns1.hukot.cz. hukot.hukot.cz. 1771230722 28800 7200 604800 1000
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0