iptables help to forward ports please

[Hanasaki JiJi]

been trying to get the following to work for sometime.... input is most 
appreciated

internet <=25= firewall iptablerule =port#x=> internalSMTPhost

how can the firewall be told to:
        take all incoming tcp port 25 traffic and send it to
        smtp host on port X
        take all outgoing traffice from smtphost <port25only>
        and send it out to the internet on port 25
Thank you.

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