mattermost.oml.RU has no MX records.
Question and response
Question
dig mattermost.oml.RU. MX
Answer
The response has no answer records.
Authority
oml.RU. 884 SOA ns.megagroup.RU. dnsmaster.megagroup.RU. 1139964 21600 1800 1209600 86400
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0