With the level of spying did by NSA, I dont think that there is any full proof way to protect the company critical information. The only way I can think off is that use encryption and de-cryption logic for converting the messages and documents into coded words. This logic should be known to your company and your important clients. But in this case also the you can protect only upto50-60% as many times we have to send the message / documents to new client / persons.
