I am thinking to order a VPS plan to act as mail server. All hosts in server A will connect to a VPS, named B, as email server. Users in server A will send mail to exim service of server A then exim forward mail to B. B will work as a gateway to send out mail. I will setting to prevent user send spam by limit mail out per hour. How can I config A and B to work? The reason is server A behind a firewall and not allow outside access to popular port. Does VPS-M plan good for this case? And how to config?