I need help to configurate courier to obtain the following functionalities:
1.  To use courier as a mail relay to our Mail Server but its remains the local traffic of our accounts.
2.  To activate courier as a mail filtering and protect our mail of virus/trojans attacks.

My environment is described for the following schema:

PC/Linux   ----\
PC/Windows ----  \
PC/Windows ----    /----------Gateway Linux ----- Cable Modem ---- Internet ------- Mail Server
.                 /           Proxy
.                /
PC/Linux   ---- /

Thanks in advance for any help.