[exim] Is there a way I can check a file for valid users?

In my setup I have a file that contains the valid users for my domain
and any mail to any other user than those should be binned immediately.

Can anybody tell me how in my .forward (note I only control .forward)
can I setup the construct to check the contents user portion of the
email address against a file containing valid users.

So $header_to: would have username@domainname

I want to check username against a file of valid users I don't want to
have to have that list in both .forward "literally" and the file
that's already maintained. I'd rather it simply use the file as it's
defined now.