Postfix Question with front end filtering [OT]

[Marc Perkel]

Sorry for the OT question but just need a quick answer from a postfix 
expert.

Here's the problem. I run a front end spam filtering service. Email from 
the world comes in, I clean it, and send the good email to the original 
server. However sometimes because my service is now the primary MX when 
I forward the good email the recipient server (running Postix) rejects 
the email (relaying denied) because it no longer things it is hosting 
the domain locally.

How do I tell postfix that it is the final destination of email for 
domains where it is not the primary MX?

Thanks in advance.