A pseudonymous remailer or nym server, as opposed to an anonymous remailer, is an Internet software program designed to allow people to write pseudonymous messages on Usenet newsgroups and send pseudonymous email. Unlike purely anonymous remailers, it assigns its users a user name, and it keeps a database of instructions on how to return messages to the real user. These instructions usually involve the anonymous remailer network itself, thus protecting the true identity of the user.
Primordial pseudonymous remailers once recorded enough information to trace the identity of the real user, making it possible for someone to obtain the identity of the real user through legal or illegal means. This form of pseudonymous remailer is no longer common.
David Chaum wrote an article in 1981 that described many of the features present in modern pseudonymous remailers.
The Penet remailer, which lasted from 1993 to 1996, was a popular pseudonymous remailer.
Read more about Pseudonymous Remailer: Contemporary Nym Servers