First you need to configure a functioning set of relays:
Exchange needs to have the postfix servers listed in the SMTP connector 'smart host' field, this means that Exchange will relay all email sent outside the organisation via the postfix servers. (outbound)
The postfix servers need to be configured to accept email for the domain that Exchange is hosting eg. example.com, and forward it to the Exchange servers, this can usually be done with the /etc/postfix/transport file.
The postfix servers need to be configured to relay mail for the Exchange servers, usually the 'mynetworks' directive. (outbound)
With proper name resolution configured, this setup should allow email to flow into and out of Exchange.
The next major piece of work is configuring filtering in the postfix system, unfortunately this is a complex process and to save my fingers I'm going to suggest a bit of reading, you could possibly start here:
I can provide you with a working configuration but I recommend you try it yourself first, you'll gain a greater understanding and you may come up with a better config