DHCP MultiWAN NAT 1:1

[schoopy]

Hi all,
Sorry if this is addressed somewhere but so far I have not been able to find where if so.

Here is the idea:
I have an alix with 2 WANs (WAN1, WAN2) and a LAN, I would like all of part (firewall) of the traffic from one WAN2 to be forwarded to a specific server (mail server).  The outgoing traffic from that server should also go through that WAN2.
I understand that NAT 1:1 is the solution. However I do not understand how one deals with DHCP assigned WAN2 IP. I do not know the IP I will get from my ISP, all I know is that I can have 5.

I this possible or not as I read it is not when you do not have several interfaces. In my case I do have serveral but how do you do it is mysterious. I have not idea what I should put in external subnet IP!

Thanks