mx6.MTS.RU has no CNAME records.
Question and response
Question
dig mx6.MTS.RU. NS
Answer
The response has no answer records.
Authority
MTS.RU. 52 SOA ns2.MTS.RU. dns.MTS.RU. 2026022202 10800 1800 604800 900
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0