@sap68:
1 + 2: what it's the goal to proxy all traffic outbound?
The spambot or trojan can't communicate directly through internet ad will be blocked?
The goal is to control all the outbound connections, ideally by not allowing any direct outbound traffic. By forcing all outbound traffic through a proxy you can manage it better, and log it all. If you install an email server and configure it to scan for (and block) spam and malware then that should stop any spambots.
By forcing all outbound email through a single, managed, email server and all other outbound traffic through a proxy (or opening a single port for for a single computer where the program can't work through a proxy) you can log everything. Once you've got those logs you have to go through them to find the problems and deal with them. It'll take time and effort to do that.