[sentinix-list] configuring sentinix to act as an smtp proxy

I am having difficulty configuring Sentinix/Postfix to act as a
spamfilter.  I have tried to use scott henderson's how to, but to no
avail.
 
Here is what I want to do:
 
MX record for domain lotsofgadgets.com points to the public IP of my
firewall [y.y.y.202]
SMTP traffic for the domain "lotsofgadgets.com" is forwarded through the
firewall[x.x.x.1] on port 25 to sentinix box [x.x.x.30] 
Scan email for virus/spam, and forward cleaned emails to exchange server
[x.x.x.31] for delivery to end users (no mailboxes will reside on
sentinix box)
 
Part of my confusion arrises from the fact that Sentinix has 2 postfix
directories, which I assume is so the /etc/postfix can receive the mail
and then pass it through mailscanner/spamassassin and then it is
delivered with the second installation /etc/postfix.mailscanner.out.  I
am unsure of how to setup the relay, and which configuration changes to
make.
 
If anyone has setup this configuration with Sentinix I would appreciate
their help.  
 
Thanks in advance
 
Dennis