mail.ISSR.RU has no CNAME records.
Question and response
Question
dig mail.ISSR.RU. NS
Answer
The response has no answer records.
Authority
issr.ru. 1099 SOA dns0.mts.ru. dns.mts.ru. 2025031801 3600 900 2419200 86400
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0