The Debian exim4 split mail configurations files in
/etc/exim4/conf.d/ are merged using the tool
update-exim4.conf. The default location for the merged configuration file is
Which has this help.
# update-exim4.conf --help
/usr/sbin/update-exim4.conf - Generate exim4 configuration files
-o|--output file - write output to file instead of /var/lib/exim4/config.autogenerated
-d|--confdir directory - read input from given directory instead of /etc/exim4
I haven't tested, but I believe this means you could replicate /etc/exim4 to some other folder and then use a command like this
update-exim4.conf -d /path_to_config_folder/ -o /path_to_test_config_file to generated a merged configuration file which you can then pass to exim using the -C option.
Or you can just copy the
/var/lib/exim4/config.autogenerated file to somewhere else, make and test your changes to the merged configuration, and then replicate those changes back to the files in