mxb.ASTRALINUX.RU has no CNAME records.
Question and response
Question
dig mxb.ASTRALINUX.RU. NS
Answer
The response has no answer records.
Authority
astralinux.ru. 1506 SOA ns1.yandexcloud.net. mx.cloud.yandex.net. 1 10800 900 604800 86400
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0