Sending an email results in: "failed to open /path/to/file for linear search"

Symptoms

When sending an email, the email is never received, and the following error is logged to /var/log/exim_mainlog where /path/to/file is an actual file path (example: /etc/exim_example_file).

1k3QXs-0006YO-3D failed to open /path/to/file for linear search: No such file or directory

Description 

This occurs when Exim is configured to perform a linear search (lsearch) for a specific file that does not exist.

Example:

@CONFIG@
domainlist exim_example = lsearch;/etc/exim_example_file

Based on the above configuration, if /etc/exim_example_file is missing, exim_example will fail when sending an email and the following will log to /var/log/exim_mainlog:

1k3QXs-0006YO-3D failed to open /etc/exim_example_file for linear search: No such file or directory

Workaround

This is typically caused by customizations to Exim's configuration. If the linear search (lsearch) is performed due to customization to Exim's configuration, either remove the customization or recreate the missing file and then run the following scripts to load the change:

/scripts/buildeximconf
/scripts/restartsrv_exim