Help to block port 25 and more

From:	<simon dot vetterli at hotelplan dot ch>
To:	<m0n0wall at lists dot m0n0 dot ch>
Subject:	Help to block port 25 and more
Date:	Mon, 18 Dec 2006 06:10:01 +0100

I have the follow situation:

No DMZ; NAT and behind an Exchange-Server. I like to block the port 25
for all the workstation in my LAN for outgoing.

How can I black my network from outside, that I don't have a
relay-function?

Kind Regards.