mx.selectsystem.cz has no CNAME records.
Question and response
Question
dig mx.selectsystem.cz. NS
Answer
The response has no answer records.
Authority
selectsystem.cz. 3245 SOA ns4.selectsystem.cz. hostmaster.selectsystem.cz. 2024092703 14400 7200 1209600 3600
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0