mx.ERTELECOM.RU has no CNAME records.
Question and response
Question
dig mx.ERTELECOM.RU. NS
Answer
The response has no answer records.
Authority
ertelecom.ru. 3600 SOA ns8.ertelecom.ru. noc.ertelecom.ru. 2026022001 86400 1800 604800 86400
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0