Mixes allow anonymous communication.
They hide the communication relation between sender and recipient and, thereby, guarantee that messages are untraceable in an electronic communication network. Nonetheless, depending on the strength of the attacker, several known attacks on mixes still allow the tracing of messages through the network. If the attacking mix delays all but one message, it can trace the message it is interested in. The special problem is that this attack is not detectable by the users as the behavior of the mix is completely legitimate. The chances of preventing such pool-mode attacks depend on how well the users can check the mixes in performing their tasks.