ns3.OPTORG.RU has no MX records.
Question and response
Question
dig ns3.OPTORG.RU. MX
Answer
The response has no answer records.
Authority
OPTORG.RU. 2737 SOA ns4.OPTORG.RU. admin.OPTORG.RU. 2025101503 3600 900 3600000 3600
Additional
. 0 OPT ; payload 4096, xrcode 0, version 0, flags 0