mail.ruweb.net has no CNAME records.
Question and response
Question
dig mail.ruweb.net. NS
Answer
The response has no answer records.
Authority
ruweb.net. 2540 SOA ns61.ruweb.net. root.ruweb.net. 2020021130 14400 3600 1209600 86400
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0