The Art of Spam Filtering – False Positives?

Over a year ago I posted some reflections on email filtering based on our experience as an ISP
Justin Mason’s latest post reflects on some of the issues that I had hinted at:
False Positive ‘Reports’ != FP Measurement
As he underlines in one of the comments:

The problem is, if you don’t receive the message text, it can be damn hard to figure out if the message you just blocked was spam or not — therefore they seem to just assume it was spam!