Normally default port for exim is 25. but most of the ISP block port 25. so it is necessary that an altername port for exim is open on server. Following are the steps to configure alternate port on cpanel/WHM installed on the server.
1) Login to WHM as root.
2) In WHM click on Main >> Service Configuration >> Service Manager
3) Now tick the check box under Exim on another port and also enter the alternate port no: in the corresponding text box.