Why does Exim behave differently after updating to cPanel v94?

Question

Why does Exim behave differently in cPanel v94? Why did some of my custom configurations break after updating to cPanel v94? Why can't I send mail when one of the recipient's email addresses is invalid?

Answer

Exim was updated to version 4.94 with cPanel v94 which added new features such as tainted data checking for added security. This may break custom configurations made for earlier versions of Exim.

Exim will also no longer send a message to any recipients if one of the recipient addresses is unreachable. This may affect the ability of mailing lists that contain invalid addresses to send mail to valid addresses.

Our documentation goes over these changes in more detail:
https://docs.cpanel.net/release-notes/94-release-notes/#exim-upgraded-to-4-94