One-to-one NAT iptables!

I'm not good at Iptables in Linux. I am trying to configure one-to-one NAT on my Linux gateway, unsuccessful so far. How do I do that?? Here is the scenario

eth2 - External interface
eth0 - Internal interface

Lets say I want to route (NAT) a virtual IP of 67.155.x.x to 192.168.1.10 for smtp service (infact for all services). How do I do that using iptables??

Thanks a lot,
Sanjay.