mail.SIBDC.RU has no CNAME records.
Question and response
Question
dig mail.SIBDC.RU. NS
Answer
The response has no answer records.
Authority
SIBDC.RU. 9195 SOA ns1.snt.RU. dnsmaster.sinor.RU. 2026021600 86400 7200 1209600 345600
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0