In an effort to bypass your email filter, spammers often send directly to your mail server. If you have a dedicated mail server, one solution to prevent spammers from doing this is to configure our service to deliver your mail using an alternate port and shutting down port 25 on your mail server.
If your users send outbound email through your mail server, make sure that they are not using port 25 (on most mail servers, they can send authenticated emails through port 465 or 587) or they will be blocked from sending outbound email.
1) Contact the administrator of your dedicated mail server and have them add a port on your mail server to accept inbound email on a port in addition to the standard incoming mail port (port 25).
2) Sign in to the control panel and go to Settings > Delivery mail host.
3) Enable the Use custom mail host port option.
4) Enter the custom port, and click Update mail host.
5) Allow up to 5 minutes for the change to take effect. Send a test message to your domain from an outside mail server to ensure that your mail host is working properly with the alternate port.
6) Finally, have your email administrator disable port 25 in your mail server software or block port 25 using a firewall rule.