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. 8605 SOA ns1.reg.RU. hostmaster.ns1.reg.RU. 1759328272 14400 3600 604800 10800
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0