mx2.mail.corp.MASTERHOST.RU has no CNAME records.
Question and response
Question
dig mx2.mail.corp.MASTERHOST.RU. NS
Answer
The response has no answer records.
Authority
corp.MASTERHOST.RU. 356 SOA ns5.MASTERHOST.RU. hostmaster.MASTERHOST.RU. 1 28800 7200 1209600 600
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0