mail.mob.ru has no CNAME records.
Question and response
Question
dig mail.mob.ru. NS
Answer
The response has no answer records.
Authority
mob.ru. 3259 SOA ispvds.com. tech.ispvds.com. 2023110931 10800 3600 604800 86400
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0