We have several Debian Etch machines running with the famous ISPConfig
using Postfix, policyd-weight, SpamAssassin, saslauth, Courier IMAP+POP3, ...
While our system proved pretty effective in regards of incoming spam mails, I am aware that today infected and otherwise compromised machines are used to send spam. What I'd like to do is check outgoing mail (from our customers and ourselves) and detect spam within them.
This would inform our customers/us if something may be wrong with a pc and may also prevent our servers from getting blacklisted (they never were, but you can never be cautious enough).
I just wonder how to achieve this.