smtp.RBC.RU has no CNAME records.
Question and response
Question
dig smtp.RBC.RU. NS
Answer
The response has no answer records.
Authority
RBC.RU. 60 SOA ns.RBC.RU. noc.RBC.RU. 2026022800 3600 1200 604800 60
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0