Alex Chitu: How Gmail Blocks Spam. In: Google Operating System: Unofficial news and tips about Google. 29. Oktober 2007, abgerufen am 12. Februar 2009: „Gmail’s filters are constantly improving and an important ingredient of their effectiveness is the use of community signals. Every time you click on the „Mark as spam“ button, Gmail uses that information to block similar future messages not only for you, but for all Gmail users also.“