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 :

 

Smtp request-->             Site B : 25 -->                Forwarded to Site
A :111 -->        Nat'd to internal Network via port 25

 

Both site use M0n0wall 1.2b as their firewall/router and are physically
located about 36miles apart. And have two totally different access
providers.

 

Now I know I can just change the port number that SMTP operates either at
the NAT level or server level, but I have about 120 customers that currently
use port 25 and I would be pulling my hair out if I had to talk to each one
of these people on the phone and walk them through changing the SMTP port.
Not very many of my clients are computer literate.