[kvetch]

Actually , I had almost the same problem , I wanted to have specific service deliver to a specific internal ip address.
To achive this you have two steps to do

1. define inbound rule for the connection : let's say your public ip is 212.1.1.1 and your internal web server is on private ip 10.1.1.1 define rule : source : any ; destination : web server (10.1.1.1) ; service:25 (smtp).
2. define NAT rule (address translation) , make it the first rule , and it's should look like this: original packet- source : any , destination : your checkpoint node(212.1.1.1) service : smtp(25)
Translated source = original , destination = internal web server (10.1.1.1) service = original.

That did the work for me.

Now , test your setup with c:\telnet 212.1.1.1 25

Hope it's work for you (it's worked for me though)