proxy+port redirection behaviour

[Metal Gear]

Hi all,

1) Dedicated Firewall
2) Server
3) Clients

It is a switched network and the 'Server' has only one interface card 
connected to the switch. The 'Server' can only access the internet as i have 
specially blocked the clients on the 'Dedicated Firewall'. Now what i want 
is that 

1) When clients connect to the smtp port of the 'Server' they are redirected 
to the another smtp server on the internet. One more thing my 'Server' is 
not listening on port 25 i just want to have a behaviour of port redirector. 
Redirecting connections destined to the 'Serve' on port 25 to a specific 
server on the internet.

2) Now in this case i wanted to have the behaviour of proxy server. Whenever 
client connects to any ftp (or any other service) server on the internet 
they are silently proxied thru the 'Server' as 'Server' can only access the 
internet.

My clients gateway is set to 'Server'

Is that possible to achieve thru iptables if yes can someone give me a 
direction or iptables script how to accompalish that.

Thanks...