If you already use web hosting from RU-CENTER but you want to use your own mail server, you need to add to your domain’s zone file an MX record pointing to that mail server and an А record pointing to IP address of the mail server.
To add such records, you need to:
The changes will take effect in 1—2 minutes.
You also need to contact your Internet service provider that provides you with an IP address for mail server with a request to create a PTR record with the name of your mail server.