Where do I edit the Exim Configuration?
There should never be a reason to edit the Exim configuration file at /etc/exim.conf directly. Changes overwrite if an Administrator writes to this file. Instead, the recommended method of editing the Exim Configuration file is through the "Advanced Exim Configuration Manager" in WHM. The interface creates a file at /etc/exim.conf.local where customizations are read. This file is also excluded from the rebuilds of the main configuration file. This way, any customizations made to Exim are kept.
We have documentation on this section of the Exim Configuration Manager if needed:
cPanel Docs: Exim Configuration Manager » Advanced Editor
You may also want to review the Exim documentation:
Exim's Docs: Chapter 14 - Main configuration
Consider subscribing to the Exim users mailing list and asking other administrators for assistance:
cPanel Technical Support does not provide direct assistance customizing Exim and we recommend such changes be reviewed by a administrator familiar with Exim.