Transparent mail proxy

[nonamey]

Can anyone tell me how to set up a transparent mail proxy? Specifically, what I want is to put a mail filter (spam/virus) on a firewall such that all mail requests are passed through it and then forwarded to the clients. The filter should be transparent-- the clients should not have to make any configuration changes and should in general not be aware of the presence of the transparent proxy.

Note that I do not want to set up a mail server on the firewall such that users inside the network have to change their mail configuration. To illustrate my point, the mail should work if the firewall is replaced by another gateway without any configuration changes on the clients (obviously functionality provided by the firewall-- spam/virus filtering-- would not be present).

Ideally, the filter would work for both incoming and outgoing mail, although incoming alone would be sufficient.

Edit: Let me further specify that the clients are using POP3 hosts and should be able to change them at their leisure.

Thank you for any help or pointing me to the correct documentation.

[magic919]

[c_riis]

How do i setup iptables for p3scan ?

i need to scan both incoming and outgoing mail with this

Thanks, Christian