sender20.mail.selcloud.ru has no CNAME records.
Question and response
Question
dig sender20.mail.selcloud.ru. NS
Answer
The response has no answer records.
Authority
mail.selcloud.ru. 57 SOA a.ns.selectel.ru. support.selectel.ru. 2026022003 10800 3600 604800 60
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0