Spam from NAT boxes

[Cesar]

Hi,

   I have some NAT boxes running FreeBSD, each of these boxes do NAT for 
like 100+ people.
   Almost everyday my IPs got blacklisted because of spam. I cant block the 
smtp traffic going out became some people need it to send true e-mails.
   Are there any tool to detect/block those spams?

   I tought in a program that receive the connection diverted/forwarded by 
ipfw and then deliver it to SpamAssassin ...

   I also have an e-mail server fully configurated with anti-spam, 
anti-virus ... I tried forward to this e-mail server all my NAT box tcp 
connections to port 25.

   ipfw add fwd xx.xx.xx.xx,25 tcp from 192.168.0.0/24 to any 25

   I got some matches in this rule when I try to send an email, but I didnt 
get redirected to my email server.


   Any ideas and/or sugestions?


Thanks