mx.dok3.ru has no CNAME records.
Question and response
Question
dig mx.dok3.ru. NS
Answer
The response has no answer records.
Authority
dok3.ru. 5171 SOA ns1.reg.ru. hostmaster.ns1.reg.ru. 1766060275 14400 3600 604800 10800
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0