mx.PROPLEX.RU has no CNAME records.
Question and response
Question
dig mx.PROPLEX.RU. NS
Answer
The response has no answer records.
Authority
PROPLEX.RU. 375 SOA ns3-l2.nic.RU. connect.proplex.RU. 2012041479 14400 3600 2592000 600
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0