In many cases, you may have experienced that standard SMTP port 25 has been blocked by users ISP. Due to this your users may not be able to send/receive emails from their email clients.
In this case, you can run exim on another alternative port something like port 26 or 587 rather than the default Port 25. Below are the exact steps do to so:
1. Log in to WHM.
2. Go to Service Manager.
3. Check the box for Exim on Another Port.
4. Set the port to 26 OR 587.
5. Click save.
Note: Please check that this port is not closed by your firewall.