Using NAT to relay traffic

[Jared Cook <jared@xxxxxxxxxxx>]

I have two servers on two different networks.  I am running a service on 
box A that I am transitioning to box B.  While I wait on DNS to 
propagate, I would like to do some iptables magic to send traffic from 
box A to box B using NAT.  For instance, when pop3 email users connect 
to box A, I would like box A to send the request to box B 
transparantly.  Is this possible?  I have had success doing port 
forwarding to the local machine, but when I specify box B as the "--to", 
it doesn't work.  Any help would be greatly appreciated.

Thanks,
Jared