mx.LONG-EDU.RU has no CNAME records.
Question and response
Question
dig mx.LONG-EDU.RU. NS
Answer
The response has no answer records.
Authority
LONG-EDU.RU. 3600 SOA vip154.hosting.reg.RU. support.reg.RU. 2022091359 10800 3600 604800 86400
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0