speedtouch 530 static IP and email server

Can anyone remember how to configure a speedtouch 530 (ver 4.2)? I have an old one spare and am thinking it would be a useful backup equipment. I have a bunch of public ip addresses from my ISP (x.x.x.15 to x.x.x.20) and should have my email server (lan address 192.168.0.1) sending email via smtp on public ip x.x.x.16.

The 530 will be backup for 2 bits of kit (modem and firewall ) which take .15 and .16 public ip addresses on their respective wan interfaces.

If i replace the kit with the 530 as is, the 530 gets the .15 ip address hence the email server is seen by outside as sending from .15.

I expect the 530 can translate the email server's address from 192.168.0.1 to x.x.x.16 for outgoing email on port 25, if i could work out how to configure it. The manual does not mention this. Any help appreciated.

in case anyone wants to know, i could not work out how to do this on st 530. however i did set it up on a zyxel p-660r - allow the zyxel to obtain the wan ip address automatically and enabling its full firewall feature. seems to be no need to create additional static ip addresses on the wan. then setup address mapping. hints provided at Full Feature NAT on Prestige Routers.

one thing i noticed upon testing was that one to one type mapping rule for the email server opened 6 odd ports through the firewall to the server. using many to one type stealthed the ports and then added a server type rule for email only.