mail.STACKSOFT.RU has no CNAME records.
Question and response
Question
dig mail.STACKSOFT.RU. NS
Answer
The response has no answer records.
Authority
stacksoft.ru. 9544 SOA ns.stacksoft.ru. admin.stacksoft.ru. 2025101503 10800 1800 604800 86400
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0