mail.SYLERONT.RU has no CNAME records.
Question and response
Question
dig mail.SYLERONT.RU. NS
Answer
The response has no answer records.
Authority
SYLERONT.RU. 1609 SOA firstvds.RU. guttaviva.gmail.com. 2023051300 10800 3600 604800 86400
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0