help with iptables

[Dave Cramer]

Heres what I want to do

I have a new mail spam filter machine I want to test before I change  
the mx records


this machine I will call A receives mail and forwards it to B

currently B is the MX

So what I'd like to do is  using iptables :

route port 25 traffic currently going to B --> A except when it comes  
from A

I tried

iptables -t nat -A PREROUTING -p tcp -m tcp -s ! A --dport 25 -j DNAT  
--to-destination A

but this didn't work

suggestions ?

Dave
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