OT: mailscanner integrated in transparent smtp proxy

[Paulo Roncon]

Hi, 
this is OT but maybe someone can help me...

I'm trying to come up with a transparent smtp proxy to filter outbound mails in a ISP like environment with mailscanner.
After a bit of googling i have smtp transparency, but not ip transparency (SPF tests fail on the receiving end)
has anyone worked in this problem?
Tproxy? wccp? any ideias?

thanks!!