Given that ISPConfig is designed to work with SpamAssassin, SA seems like the obvious choice. Are you not already using it on your ISPConfig server?
Then if my clients complain that they aren't getting emails from someone I can adjust postfix accordingly.
Your clients should always receive mail, even if it is almost positively SPAM. That's what a "Junk" folder is for. Deleting mail automatically is strongly discouraged, and if I'm not mistaken, illegal in some places.
Also, don't get into the business of adjusting anything manually (beyond basic SPAM Policy configuration). The most effective way to combat SPAM (in my experience) is with a well-trained Bayes (Bayesian) database. SpamAssassin uses a Bayes database, and the fastest (and arguably most effective) way to train it is to let your users do the training.
If your server uses Dovecot to handle incoming mail, you can use the Antispam Plug-in, which enables your users to drag mail from Inbox to Junk and train the Bayes database in real-time. Conversely, for false-positives, users can drag messages from Junk to Inbox, and the Bayes database will be trained accordingly.
If your server uses Amavis to interact with SpamAssassin (my preferred setup), you can use the amavis-stats
package to generate comprehensive graphs (generated with rrdtool) regarding SPAM, viruses, etc.
Finally, you can use a tool like logwatch
to receive daily digests that include a detailed list of how every message was handled ("Stored to Inbox", "Stored to Junk", etc.).
I haven't read through the post that you cited, so I don't know how this advice compares; take it for what it's worth .