A cancelbot is an automated or semi-automated process for sending out third-party cancel messages over Usenet, commonly as a stopgap measure to combat spam.^[1]

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One of the earliest uses of a cancelbot was by microbiology professor Richard DePew, to remove anonymous postings in science newsgroups.^[2] Perhaps the most well known early cancelbot was used in June 1994 by Arnt Gulbrandsen within minutes of the first post of Canter & Siegel's second spam wave,^[3][4] as it was created in response to their "Green Card spam" in April 1994.^[5] Usenet spammers have alleged that cancelbots are a tool of the mythical Usenet cabal.

Cancelbots must follow community consensus to be able to serve a useful purpose, and historically, technical criteria have been the only acceptable criteria for determining if messages are cancelable, and only a few active cancellers ever obtain the broad community support needed to be effective.

Pseudosites are referenced in cancel headers by legitimate cancelbots to identify the criteria on which a message is being canceled, allowing administrators of Usenet sites to determine via standard "aliasing" mechanisms which criteria that they will accept third-party cancels for.

Currently, the generally accepted criteria (and associated pseudosites) are:^[6]

By general convention, special values are given in X-Canceled-By, Message-Id and Path headers when performing third-party cancels. This allows administrators to decide which reasons for third-party cancellation are acceptable for their site:

• The $alz convention states that the Message-Id: header used for a third-party cancel should always be the original Message-Id: with "cancel." prepended.
• The X-Canceled-By: convention states that the operator of a cancelbot should provide a consistent, valid, and actively monitored contact email address for their cancelbot in the X-Canceled-By: header, both to identify the canceler, and to provide a point of contact in case something goes wrong or questions arise regarding the cancelbot's operations.
• The !cyberspam convention states that specific pseudosites should be given within the cancel message's Path to identify them as complying with certain cancel criteria, see above.

• Dave the Resurrector