A “Smart Host” (also known as a relay host) is a type of mail relay server which allows an SMTP server to route e-mail to an intermediate mail server rather than directly to the recipient’s server. This improves the performance of your server and the reliability of your e-mail delivery.
Exim Server (Forwarding Server)
This server will be forwarding all emails to the SmartHost for relay for final delivery. If you are not using cPanel / WHM, then you will simply edit ‘/etc/exim.conf’ instead of using cPanel / WHM’s interface. However, if the server is running cPanel / WHM you must edit the configuration file through the GUI as cPanel / WHM will revert your changes automatically over time.