On linux or BSD, you can easily set some bridge, at layer 2 level, and have it analyse all packets, with some netfilter/ipfilter/whatever rule to handle locally SMTP/whatever traffic.What you describe is a context based access control firewall, wich is very diferent than what Stephano described.
This even sounds like a very good idea,
Stephano, good idea. I think I'll test this some day :)
With a context based firewall you can analize and filter INDEVIDUAL packets based on access control lists. This is _VERY_ different than analizing email messages for spam or virii.
