> In my LAN network if any host send anything to a *particular External IP
> Address*[* A.B.C.D*] and *port 25 *=> I have to send to the Same IP and
> Different port out [* A.B.C.D*] and *port 26*

> I could not find any place to make this port rewrite for the internal
> network.

I've seen a similar thing (ip address change instead of port change)
on a similar platform (pfSense instead of m0n0wall).

I suppose that port redirection would work about as well the same as
ip address redirection. But I don't know if it'll work with m0n0wall.
While pfSense is a m0n0wall fork, it used (AFAIR) pf instead of
iptables, so it might not work. However, if your hardware is powerful
enough (and you're not afraid about being overwhelmed by the excessive
number of function in pfSense), you might consider a migration to
pfSense, if this solves you problem.

Okay, here's the tutorial I stumbled upon last week:

http://www.interspective.net/2012/07/pfsense-ntp-and-network-sneakery.html

No, i didn't try it myself - I was looking for something totally
different. However, I read the article anyway, just out of some sort
of, um, "geeky fascination".

Best regards, Klaus