Howto forword email messages with Exchange 2003 to an external smtp server when the domain is registered with Exchange 2003, but the email address is not?

Hi,

I am having difficulty in finding the right words to put into the search fields to get the right results on what I am looking for. But I will do my best to clarify my problem...

This is the situation:

Using the Internet Connection wizard in Windows 2003 SBS Premium, I configured Exchange 2003 to use the domain planet.nl. I am using a POP3 connector to get my email form the POP3 box at my ISP. I configured Exchange to send email through the smarthost of my ISP (This is necessary, because my ISP blocks direct acces to port 25 to prevent / control spamming. But this is not the issue).

This is the problem:

I do not own the domain planet.nl. It is the domain of my ISP. I only use a couple of email addresses which are in the domain planet.nl. Exchange thinks I own the whole domain. So when I send an email to somebody else on the domain planet.nl, but who's address is not registered at my Exchange server. Exchange returns the message saying the address does not exist on the domain (which I understand). Messages to other domains are handled perfectly.

This is the question:

How can I make my Exchange server forward an email to the smarthost of my ISP when the address is in the same domain as my Exchange server is, but when the address itself is not registerd with my Echange server?

Really looking forward to an answer! I guess the solution is pretty easy?! But as I said, I am having problems in finding the right words to feed Google, etc.

Thanks in advance!

Regards,

Wim