Carnivore is one such tool, intended to catch criminals by monitoring their e-mail messages.
Encryption using public or private keys can protect messages from packet sniffers.
Once a message is received and decrypted, there s still a chance it can fall into the wrong hands or be confiscated by law enforcement agents should they have a court order. Encryption keys are stored on a Disappearing Access server, and a local key cache lets you read secure messages off-line. Taking a minimalist approach, Disappearing Email still permits copying and forwarding of secure messages. Opting for a much more stringent perspective, SafeMessage uses a standalone, secure mail client and a proprietary transport protocol, essentially eliminating message persistence by avoiding e-mail servers and less secure protocols altogether. SafeMessage uses multiple layers of encryption and requires validation of both parties, confirming the identity of each.