[exim] I need to filter the rcpt field

Hello:

I have exim4 installed on debian, this server is the smarthost for a domain
controlled internally on the network by an exchange server.

I have the whole list of available accounts on the domain, and I want to
configure exim4 to pass to the exchange server only the messages with valid
rcpt field.

I have tried to configure by myself without the result I want to get.

On the other hand, I want to be sure that exim4 is making a dns lookup to
all the mailers that are trying to send emails to this domain.

Thanks in advance for any help,

Enediel
Linux user 300141
Debian GNU/Linux