mx.tver.ertelecom.ru has no CNAME records.
Question and response
Question
dig mx.tver.ertelecom.ru. NS
Answer
The response has no answer records.
Authority
tver.ertelecom.ru. 3600 SOA ns8.ertelecom.ru. noc.ertelecom.ru. 2026020503 1800 180 1209600 86400
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0