With most ISP’s blocking port 25 for all smtp servers besides their own, you may want to allow cPanel\EXIM to run on port 587. All you need to do are the following steps:
1. Login to WHM as root user.
2. Select Service Configuration -> Exim Configuration Editor
3. At the bottom of the page, click on the Advanced Editor button.
4. Add the following code to the very first text box.
daemon_smtp_ports = 25 : 587
5. Click Save at the bottom of the page.
That’s it, you now have the ability to send email via port 25 or port 587 on your server.