question about using iptables.

[Nicolas Grossi <nicolas.grossi@gmail.com>]

People:
           I´m new using iptables and i want to know if it is possible 
to forward all the packages to a specific port to a specifica ip 
address. This is my environment and what i need to do:

I have a lan under 190.168.0.x

then a have a gateway with iptables + ipcop to filter all packages to 
WAN (internet)

My ISP give us a set o public ips and all the packages from the lan to 
the wan use one public ip (one from that set) = 190..139.106.194

Currently our smtp server is been considered as SPAM for a few domains 
(and we need to change this ASAP).

It is poosible to configure iptables to set all the packages from 
190.168.0.x:25 (smtp port) to use a different public ip addres of the 
set that our isp gives us?

brieflly:

all packages from our lan to a port different to 25 use 190..139.106.194 
and all packages from our lan to port 25 use 190.139.106.195

Thanks in advance!

Nico