mxfront01.rt-dc.RU has no CNAME records.
Question and response
Question
dig mxfront01.rt-dc.RU. NS
Answer
The response has no answer records.
Authority
rt-dc.RU. 16 SOA ns1.ngenix-dns.net. noc.ngenix.net. 1 7200 600 1209600 300
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0