SUMMARY: Debian 3.1: IPTables Rule

First off, thanks to all who replied:

Frank Smith, Sean Ryan, Cary Penniman, and Binand.

I should have been more clear about my setup, which is that the mail 
server is behind the firewall. The purpose for this rule is to block 
mass amounts of e-mail from sending out from virus infected laptop 
users. This happened and as a result got our IP banned from AOL, Yahoo, 
and AT&T. :-( Now with the rule in place, I don't have to worry about 
that situation recurring...or at least I can worry less.

Ultimately, I used Frank's solution (which works perfectly) but with 
DROP instead of REJECT on the last rule. In his words:

"At a minimum, all you should need to add would be the FORWARD rules I
originally gave you. plus an 'established' rule.  Order does matter, the 
first rule that matches a packet controls what happens to it.

# allow established connections (allow replies to allowed packets)
iptables -A FORWARD -m state --state ESTABLISHED -j ACCEPT
# allow your mail server to send mail
iptables -A FORWARD -p tcp -s 192.168.1.111 --dport 25 -j ACCEPT
# also allow it to receive mail
iptables -A FORWARD -p tcp -d 192.168.1.111 --dport 25 -j ACCEPT
# reject all other SMTP connections
iptables -A FORWARD -p tcp --dport 25 -j REJECT

This assumes you have 'state' match support (which you probably do since
I think it is required to do NAT in iptables)."

Thanks again!

Chris Fischer
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