Generally considered the father of anonymous communications, David Chaum first proposed a system for anonymous digital communications in 1981. The system he proposed used a special mail server, called a Mix, to process email.
A Mix is a computer that mediates between senders and recipients. A Mix is a store-and-forward device that accepts a number of fixed-length messages from numerous sources, performs cryptographic transformations on the messages, and then forwards the messages to the next destination in an order not predictable from the order of inputs.
MIX enables anonymous communication by means of cryptography, scrambling the data, and unifying it (padding to constant size, fixing a constant output rate by sending dummy data, etc.).
Chaum Mixes support sender anonymity, and protect from traffic analysis. a Mix looks like a black box with myriad inputs and outputs. As long as the integrity of the box is assured, tracking a specific connection through the Mix is a difficult challenge.