mx.siel.si has no CNAME records.
Question and response
Question
dig mx.siel.si. NS
Answer
The response has no answer records.
Authority
siel.si. 1615 SOA dns1.siel.si. dns.siel.si. 1302 28800 7200 604800 86400
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0