An anonymous remailer is a service that allows the sender of an e-mail to hide the source of the e-mail. The anonymous remailer strips the return address information from a message, adds a computer-generated code as a return address and then remail the message from its own server.
The effect of doing so is that the recipient receives a message from an anonymous user name at the anonymous remailer’s domain. In many cases, the anonymous remailer maintains the original return address information so that any return mail may be forwarded to the original sender.
For additional security, messages may be forwarded from one anonymous remailer to another, or several, before delivery to its ultimate addressee. Messages can also be encrypted in layers so that in each step along the chain of anonymous remailers only the next recipient is known. While still not foolproof, the discovery of a particular sender’s identity would require disclosure by multiple anonymous remailers.
In the alternative, anonymous remailers that do not retain any of the original message information can provide an even higher level of security. Discarding the original message information, however, precludes return mail to an anonymous sender or assignment of the same anonymous address to a sender’s subsequent messages.
There are several types of remailers, including:
- Type‐0 Remailers are the oldest and simplest systems for e‐mail anonymity. The message goes from the sender to the remailer, who strips the identity of the sender and remails the message to the recipient. A pseudonym is assigned to the sender which is recorded.
- Type‐I Remailers: are based on the same principle, but with a number of improvements, such as: "chaining" (using a chain of multiple, independent remailers), encryption and "mixing" (incoming messages to a remailer are batched together and randomly reordered before they are sent out).
- Type‐II Remailers address problems presented by Type‐I remailers, specifically their susceptibility to size correlation attacks and reply attacks. To defeat size correlation attacks, Type‐II remailers divide all messages into several fixed‐sized packages which are sent separately through the network of remailers. More complex techniques are used to resist reply attacks.
- Type‐III Remailers include a better system for handling replies to anonymous messages and provide improved protection against reply attacks.