Remember that they had the information relating to the 9/11 incident but had not by the time of the attack actually got around to listening and categorizing the data that they already had. That only became Important after the fact and if it had of had the resources to deal with the Daily Data Collected they may have been able to prevent it from happening but like the Do More with Less that seems so popular today they have the means to collect the data but not the resources to collate it to prevent things happening unless they are actively investigating that action before the event.
With Remailers they know who is sending stuff to these services, the time delay between receiving and sending and what is sent from them just not who is responsible for what individual items are forwarded though with enough time they can effectively work out who is most likely to be responsible.
The reason why these services are so difficult to track is because they lack the Priority needed to make it possible to to track things and this is what effectively makes the original sender anonymous.
Of course if someone is interested enough and has the Technical Resources at their disposal they can eventually track things down but the issue here isn't the Technical Possibility of Tracking the Perp Down but the Proper Authorities doing the work. Like most Intelligence Organisations Data is not shared and many times the same systems are duplicated with the same results of Masses of Data being collected but very little of it being actually looked at to see the level of threat if any that it posses soon enough.
A perfect example of this is a Invoice sent via E Mail that says something along the lines of Used Boot & Nuke to wipe the HDD and then reloaded. "Nuke" being a Key Word which is Flagged results in all E Mail with it in it being copied and then at some time in the future to be determined by the Powers that Be actually looked at to see what is actually being talked about.
This isn't even something new but has been around since National Telephone Systems where first introduced. It's just got more Technical not something new happening.
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