mx1.RUSENDER-MAIL.RU has no CNAME records.
Question and response
Question
dig mx1.RUSENDER-MAIL.RU. NS
Answer
The response has no answer records.
Authority
RUSENDER-MAIL.RU. 44 SOA a.ns.selectel.RU. support.selectel.RU. 2026020302 10800 3600 604800 60
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0