Filtering Techniques

• server level
• reader level

• Permanent race: spammers/filters, virus/antivirus
• New filters needed all the time to keep efficiency reasonable

• social: law enforcement, bandwidth cost
  
• based on the sender/last hop
• blacklists: database queries based on
  
• nameserver request (Open Relay DataBase, five-ten-sg.com, Spamhaus, Spamcop, Relay Stop List,  many others) <- targeted by viruses DOS
  
• specialized software (Vipul's Razor)
  
• whitelists: centralized databases, challenge/response systems
  
• based on the contents
• static: keywords, style, disclaimer, features, signatures (procmail, antivirus software, readers built-in features)
  
• dynamic: bayesian filters, training (readers built-in features, bogofilter, SpamBayes, POPFile, SpamTUNNEL, PASP)
• combination of the above (SpamAssassin)
• exotic: single-use e-mail addresses