file that exists is used. Failure to open an existing file stops Exim from
proceeding any further along the list, and an error is generated.
-The file names need to be absolute names.
-
When this option is used by a caller other than root, and the list is different
from the compiled-in list, Exim gives up its root privilege immediately, and
runs with the real and effective uid and gid set to those of the caller.