The key thing that worries me about it is even if no-one reads all those emails that are stored, what if they are mined for data and used to make predictions?

The key thing that worries me about it is even if no-one reads all those emails that are stored, what if they are mined for data and used to make predictions? Last.fm can guess the type of music I like about 25% of the time, Google can guess the type of information I'm interested in around 70% of the time (figure based upon potentially ambiguous web searches I do). Neither of those services have very much metadata from me about their respective subject areas. If the NSA/GCHQ/5 eyes are hoovering up all this metadata about pretty much everything I do online, that's a ton of information to start mining for patterns - whilst legitimately say that no employees are reading it. What sort of predictions can they make? What's the accuracy of it? When do they start acting on the predictions thrown up by the system? And who polices that?