The goal: To provide a system which will flag the most spam with the least 'collateral damage' (flagging of wanted email). This is obviously a never-ending and difficult task. This system is driven by people (some would say "mob-rule"). As such, it is intelligent but also faulty. People are instructed to report only email which is UBE (Unsolicited Bulk Email). Users being what they are, they may accidentally or maliciously report other email. So the results are fuzzy, but surprisingly accurate and agile (try it yourself - the results speak for themselves).
The description that follows becomes increasingly complex. It is an attempt to spell out accurately the increasingly complex rules that SpamCop uses. This is done to avoid any confusion on the part of those using the list to filter email and those who wind up being blocked by it. However, some rules are spelled out in vague terms to slow down spammer adaptation.
This blocking list is somewhat experimental and should not be used in a production environment where legitimate email must be delivered. It is growing more stable and is used by many large sites now. However, SpamCop is aggressive and often errs on the side of blocking mail - users should be warned and given information about how their mail is filtered. Ideally they should have a choice of filtering options. Many mailservers can operate with blacklists in a "tag only" mode, which is preferable in many situations.