Just to see if I could, I tried this.
Although relatively simple to hack in a feature to do this through the
webgui, I didn't take the time to do it.
Goto exec.php, and execute 'ipnat -l'. Copy and paste starting from the
line after "List of active MAP/Redirect filters:" to the first blank
line into your editor. Insert a new line between the map and rdr lines
in your editor, and type the following into it:
rdr sis0 0.0.0.0/0 port 25 -> 126.96.36.199 port 25 tcp
replace sis0 with your actual LAN interface and 188.8.131.52 with the IP of
the desired IP. Save the file to your computer, and remember what you
called it. Goto exec.php, and upload the file you just created, and
then execute 'ipnat -C -f /tmp/myfile.txt', replacing myfile.txt with
the name of the file you just uploaded.
Now telnet from an internal lan computer to any IP port 25, and you
should get redirected!
If enough ppl want this, let me know and I'll see if I can't make it