mx1.RT-SOLAR.RU has no CNAME records.
Question and response
Question
dig mx1.RT-SOLAR.RU. NS
Answer
The response has no answer records.
Authority
RT-SOLAR.RU. 93 SOA ns1.selectel.RU. support.selectel.RU. 2023060256 10800 3600 604800 300
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0