mail.renrest.ru has no CNAME records.
Question and response
Question
dig mail.renrest.ru. NS
Answer
The response has no answer records.
Authority
renrest.ru. 4736 SOA ns1.reg.ru. hostmaster.ns1.reg.ru. 1759328272 14400 3600 604800 10800
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0