@redvers, thanks for your PR! By analyzing the history of the files in this pull request, we identified @4z3 to be a potential reviewer.

The config option is for a string with the actual configuration by looking at its help text:

Verbatim Exim configuration. This should not contain exim_user,exim_group, exim_path, or spool_directory.

The type of the option should probably be lines instead of string, though.

So 👎 for me on this one.

@redvers you can achieve the same effect by using:

services.exim.config = ''
  .include /path/to/configuration
'';

or

services.exim.config = ''
  .include ${/etc/nixos/exim.conf}
'';

if you want rollback support for your configuration (the path will be copied to /nix/store in this case).

@fpletz, .config is unfortunately used inconsistently throughout the repo.

I quick (unscientific) browse through the docs shows it used as a filename, application configuration literal contents, NixOS modules, expressions... and so on.

Guess I'm used to seeing extraConf for literals and .config for files or expressions.

Maybe a series of docpatches for docs which are not explicit would be useful.

Thanks for looking at the PR, enjoy your day.

@redvers this is true though. We could make extraConfig standard for literal strings and configFile for paths.