mx.siberia.net has no CNAME records.
Question and response
Question
dig mx.siberia.net. NS
Answer
The response has no answer records.
Authority
siberia.net. 163 SOA a.ns.avantel.ru. linux-admins.avantel.ru. 2024112601 10800 3600 604800 10800
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0