An anonymous remailer is a service allowing people to send email with their addresses anonymized. A number of systems can be used to provide such services, with varying degrees of anonymity and success. People may use anonymous remailers for any number of reasons, ranging from concerns about reporting things like human rights violations with a traceable email to wanting to harass people with less of a chance of being caught. Numerous services can be found online, with some being invitation-only to restrict activities.
The simplest version of an anonymous remailer contains a database of users with randomly assigned anonymous addresses like anon-12345@remailer.com. When people want to send email through the service, they send email like they normally would, and the anonymous address shows up at the 'from' line. The recipient can reply to the email and the remailer will route the reply through the database, matching username to real email and passing it on to the original sender. This type of anonymous remailer requires some trust, as the person running the service could publicize the database, exposing users to risk.
Other anonymous remailers use cryptography, often in layers, to provide more security. Some services bounce an email between multiple locations, stripping origin information out at each one. This provides more security and eliminates the database with user information; users send encrypted emails and the server decrypts them to strip sender information and pass them on, leaving no record behind. The more complex the service, the greater the anonymity, as while someone might be able to crack one server in the daisy chain, the chances of tracing an email all the way from sender to final recipient are very low.
The anonymous remailer service can be useful for a wide variety of people. Whistleblowers and other people who want to pass on confidential information may need to be able to make contact without revealing their identity for safety, and such services can be very helpful. Likewise, some people want to remain anonymous for safety while writing about political topics or other potentially contentious matters, and may use such services to interact with people without endangering themselves. Uses for anonymous remailers can also be more malicious in nature, although some servers attempt to restrict this activity to avoid subpoenas and other law enforcement activity.
While the process behind an anonymous remailer can be complicated, many are set up to be very user friendly and people can send emails anonymously with relative ease by following the directions on a remailer's website.