staging.OML.RU has no MX records.
Question and response
Question
dig staging.OML.RU. MX
Answer
The response has no answer records.
Authority
staging.OML.RU. 891 SOA ns.megagroup.RU. dnsmaster.megagroup.RU. 993036 21600 1800 1209600 86400
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0