[exim] smarthost conditions

Hello,

I'm fairly novice when it comes to EXIM, but I've successfully been able to
setup a smarthost on my web server to forward mail to another system. I'd
like to further secure this by adding conditions, if possible, so that it
will only forward if the recipient address is listed in a file. If its not
listed it can go through the remaining routers for delivery or failure.

Can this be done? Thanks!