[exim] Rejecting mail for invalid users?

As a user I can only change .forward file for exim and anything else
that's recognised for the user

I don't have access to the system wide config

At the bottom my .forward I have something like this

pipe "script.pl -runsa"

finish

....

within script.pl I have something like


!grep (/^$quoted_user$/i, `cat $user_file`) ))
{

....

so a file exists with the known users for my domain ...

is there anyway I can shift this functionality to .forward to avoid
the perl invocation and
is it possible to use the same file. I don't want to list each user
in a regex as I have a
process to maintain that list remotely.