how to configure postfix to send email out for a hosted domain

[nox171]

Hiya all,

i'm having a problem with my Suse server.
I've installed on it ISPConfig with several websites.

Also i've an OScommerce installation running on it...
Let me describe my scenario:

a user is buying something on the shop.
When the purchase is done, oscommerce send a confirmation email to the user.
Everything is fine but not if the user's email domain is hosted on the server.

ie:
demouser@externalhost.com purchase something -> email sent correctly.
demouser@hosteddomain.com purchase something -> the postmaster (myself) receive a bouceback like this:

 The e-mail system was unable to deliver the message, but did not report a specific reason.  Check the address and try again.  If it still fails, contact your system administrator.
            < server1.xxxxxxx.com #5.0.0 X-Postfix; unknown user: "demouser">

I think the mailserver is not sending the email because the email domain address is hosted on the server.
The point is that the domain has not the mailserver on my server but the client is managing it. The MX server is outside the ISPConfig server...

I hope everything is clear...
How can I configure postfix to look outside the server and send the email??

thanks
/nox