Due to the number of spammers using port 25 to send emails, many large ISPs has blocked this port from their network. This means if your client is behind their network and try to connect to your company mail server on port 25 to send email, it will be blocked. We can’t ask the ISP to unblock it, so our solution is to create an additional port on our server so that our client can connect to that port instead of 25 to send email.

If you are using WHM, it’s very easy, follow these steps below:

Login into your server’s WHM (https://[main server IP address]:2087)
In the find field search for Service Manager and click on it.
On the exim on another port line configure the port for exim to run on (for this guide, 26).
Now but a check in the enable box next to exim on another port and click Save at the bottom of the page.