Setting up an internal mail filter

[Jeff O'Brien]

First post to gnhlug....thanks ahead of time

I have a linux server (firewall(netfilter), gateway, mail(sendmail), 
internal dns) with 1 static IP(MX record too), another linux server that 
I want to act as mail filtering box(SA/clam-AV, etc..) on the internal 
side of the firewall (both WBEL3rc1).

Is there a way to have mail that comes in to the external linux mail 
server to be sent off to the internal mail server for filtering then 
back to the mail server where end users can then get their mail to avoid 
reconfiguring each client and without causing mail loops? 

-The first step would be to get the mail from the ext-mail server to the 
internal filter.
would I use a procmail rule, sendmail mailertable or firewall rule? 
-have mail accepted on filter box
-filter and score mail and send back to ext-mail server(which all 
clients are config'd to, or  change internal dns records for mail to 
point to filterbox instead of ext-mail server)

any suggestions and help are more than welcome  :-)

-- 

Jeff O'Brien