mx1.sib.mts.ru has no CNAME records.
Question and response
Question
dig mx1.sib.mts.ru. NS
Answer
The response has no answer records.
Authority
mts.ru. 900 SOA ns2.mts.ru. dns.mts.ru. 2026030310 10800 1800 604800 900
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0