From: "Brett Woodruff" <brett at skyways dot us>

> Here is a little problem I'm running into.  It'll take a little space to
> explain it...  I want to run a Mail server at "site A", but my ISP blocks
> SMTP port 25. But at another location I have the ISP doesn't block port 25
> (site B). I don't want to move the server to (site B) because I want to 
> keep
> all my servers in one place, and as close to me as possible.(my
> Garage/Shop), for easy/fast repair if needed. What I'm wanting/trying to 
> do
> is somehow like NAT when a service is requested from (site B) via port 25 
> it
> is forwarded to another public IP from a totally different IP block, say 
> via
> port 111.  like so :

This is a config snipit that some have used to forward all mail to a server 
in the DMZ.  A little tweak may do what you wish.

<nat>
 <rule>
  <protocol>tcp</protocol>
  <external-port>25</external-port>
  <target>192.168.1.5</target>
  <local-port>25</local-port>
  <interface>wan</interface>
  <descr>redirect SMTP to WAN SMTP server</descr>
 </rule>
 </nat>

                        Lee