mx1.mail.corp.masterhost.ru has no CNAME records.
Question and response
Question
dig mx1.mail.corp.masterhost.ru. NS
Answer
The response has no answer records.
Authority
corp.masterhost.ru. 600 SOA ns5.masterhost.ru. hostmaster.masterhost.ru. 1 28800 7200 1209600 600
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0