[Exim] only accept local users (for seval networks)

I'am seraching for a solution.

I wanna configure exim (3.33) (for serval networks) to only accept
SMTP-conections if the sender is a local one, without having
a SMTP-Auth(!).
Is there a build in feature or a smart combination.
(I could think of nothing to say ...)

If there isn' t something simple, I would write a director, which only
runs at verification time.
(Or is there a better solution?)


- oliver