nsa.RETN.RU has no MX records.
Question and response
Question
dig nsa.RETN.RU. MX
Answer
The response has no answer records.
Authority
RETN.RU. 7200 SOA nsa.RETN.RU. noc.retn.net. 2026022601 7200 3600 1209600 14400
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0