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