The Problem: I need to forward port 25 (smtp) from eth4 (146.145.26.90) to
             eth0 (192.168.0.200) on the mail server. The mail server must
             see the origin of the connections, so I can't chage the 
             original src.

           **************
    *******              ********
****          INTERNET           ****
    *******              ********
     |     **************       |
     |                          |
     |- eth4                    |- eth2
     |  ip:146.145.26.90        |  ip:216.164.162.139
     |  gw:146.145.26.89        |  gw:216.164.162.137
     |                          |
    --------------------------------
    |     *Firewall Box*           | - eth3 ip:192.168.132.1
    | Default GW: 216.164.162.137  |-------------------------> To Wireless network
    --------------------------------                           ip:192.168.132.100-199
      |- br0 (Bridge of eth0 and tap0)                         gw:192.168.132.1
      |  ip:192.168.0.1
      |
      |
      \       *****
        ******     *******
      ** Internal Network ** ------------------> To all workstations
        ******     *******                       ip:192.168.0.100-199
              *****                              gw:192.168.0.1
              |
              |
              |- eth0
              |  ip:192.168.0.200
              |  gw:192.168.0.1
           ------------------------
           |    *Mail Server*     |
           |                      |
           ------------------------
# iptables-save -c
# Generated by iptables-save v1.3.6 on Tue Sep 25 09:50:37 2007
*mangle
:PREROUTING ACCEPT [2937928:2460714734]
:INPUT ACCEPT [81238:17278997]
:FORWARD ACCEPT [2861996:2444201369]
:OUTPUT ACCEPT [76212:8962498]
:POSTROUTING ACCEPT [2938006:2453151127]
COMMIT
# Completed on Tue Sep 25 09:50:37 2007
# Generated by iptables-save v1.3.6 on Tue Sep 25 09:50:37 2007
*nat
:PREROUTING ACCEPT [42016:2557133]
:POSTROUTING ACCEPT [20150:1296783]
:OUTPUT ACCEPT [10228:453394]
[0:0] -A PREROUTING -s 192.168.132.0/255.255.255.0 -p tcp -m tcp --dport 53 -j DNAT --to-destination 192.168.0.200
[0:0] -A PREROUTING -s 192.168.132.0/255.255.255.0 -p udp -m udp --dport 53 -j DNAT --to-destination 192.168.0.200
[4:240] -A PREROUTING -d 146.145.26.90 -p tcp -m tcp --dport 25 -j DNAT --to-destination 192.168.0.200
[2:120] -A PREROUTING -d 146.145.26.90 -p tcp -m tcp --dport 10001 -j DNAT --to-destination 192.168.0.200
[0:0] -A POSTROUTING -s 192.168.132.0/255.255.255.0 -d 192.168.0.200 -o br0 -p tcp -m tcp --dport 53 -j SNAT --to-source 192.168.0.1
[0:0] -A POSTROUTING -s 192.168.132.0/255.255.255.0 -d 192.168.0.200 -o br0 -p udp -m udp --dport 53 -j SNAT --to-source 192.168.0.1
[144:8424] -A POSTROUTING -s 192.168.0.0/255.255.255.0 -o eth2 -j SNAT --to-source 216.164.162.139
[0:0] -A POSTROUTING -s 192.168.132.0/255.255.255.0 -o eth2 -j SNAT --to-source 216.164.162.139
COMMIT
# Completed on Tue Sep 25 09:50:37 2007
# Generated by iptables-save v1.3.6 on Tue Sep 25 09:50:37 2007
*filter
:INPUT DROP [0:0]
:FORWARD DROP [0:0]
:OUTPUT DROP [0:0]
:RULE_2 - [0:0]
[510:44054] -A INPUT -m state --state RELATED,ESTABLISHED -j ACCEPT
[1730:106901] -A INPUT -i br0 -m state --state NEW -j ACCEPT
[190:80864] -A INPUT -i eth2 -m state --state NEW -j ACCEPT
[0:0] -A INPUT -i eth3 -m state --state NEW -j ACCEPT
[0:0] -A INPUT -i eth4 -m state --state NEW -j ACCEPT
[0:0] -A INPUT -s 192.168.0.0/255.255.255.0 -m state --state NEW -j ACCEPT
[0:0] -A INPUT -s 192.168.132.0/255.255.255.0 -m state --state NEW -j ACCEPT
[0:0] -A INPUT -j RULE_2
[35551:19660073] -A FORWARD -m state --state RELATED,ESTABLISHED -j ACCEPT
[196:15605] -A FORWARD -i br0 -m state --state NEW -j ACCEPT
[0:0] -A FORWARD -o br0 -m state --state NEW -j ACCEPT
[0:0] -A FORWARD -i eth2 -m state --state NEW -j ACCEPT
[0:0] -A FORWARD -o eth2 -m state --state NEW -j ACCEPT
[0:0] -A FORWARD -i eth3 -m state --state NEW -j ACCEPT
[0:0] -A FORWARD -o eth3 -m state --state NEW -j ACCEPT
[0:0] -A FORWARD -i eth4 -m state --state NEW -j ACCEPT
[0:0] -A FORWARD -o eth4 -m state --state NEW -j ACCEPT
[0:0] -A FORWARD -s 192.168.0.0/255.255.255.0 -m state --state NEW -j ACCEPT
[0:0] -A FORWARD -s 192.168.132.0/255.255.255.0 -m state --state NEW -j ACCEPT
[2:80] -A FORWARD -j RULE_2
[2257:148686] -A OUTPUT -m state --state RELATED,ESTABLISHED -j ACCEPT
[0:0] -A OUTPUT -o br0 -m state --state NEW -j ACCEPT
[0:0] -A OUTPUT -o eth2 -m state --state NEW -j ACCEPT
[0:0] -A OUTPUT -o eth3 -m state --state NEW -j ACCEPT
[0:0] -A OUTPUT -o eth4 -m state --state NEW -j ACCEPT
[0:0] -A OUTPUT -s 192.168.0.0/255.255.255.0 -m state --state NEW -j ACCEPT
[0:0] -A OUTPUT -s 192.168.132.0/255.255.255.0 -m state --state NEW -j ACCEPT
[5:200] -A OUTPUT -j RULE_2
[7:280] -A RULE_2 -j LOG --log-prefix "RULE 2 -- DENY " --log-level 6
[7:280] -A RULE_2 -j DROP
COMMIT
# Completed on Tue Sep 25 09:50:37 2007