cPanel uses the Exim mailserver, so you'll need to to configure your outbound SmartHost/RelayHost using the Exim Configuration Manager on your cPanel server.
1. To configure Exim, open the cPanel WHM and navigate to the Exim Configuration Manager in the Service Configuration section of the sidebar men:
2. Before you make any changes, be sure to back up the current Exim configuration in the Backup tab.
3. Select the Advanced Editor tab. Search for the text “POSTMAILCOUNT", and enter the following into the text box:
driver = manualroute
domains = !+local_domains
transport = remote_smtp
route_list = * outbound.mailroute.net::25
If you want to know more about the format of the route_list line, take a look at the Exim manualroute documentation.
4. Finally, don’t forget to scroll to the bottom of the Exim Advanced Editor, where you’ll find the save button. Once you’ve done that, all email originating from your server is relayed through the Smarthost.