Chitu, Alex (29 de octubre de 2007). «How Gmail Blocks Spam». Google Operating System: Unofficial news and tips about Google. Consultado el 12 de febrero de 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. »
«Spam - Gmail Help». Gmail Help. Consultado el 12 de febrero de 2009. «If you find that some senders' messages are consistently being mislabeled as spam, you can prevent this by... »
Nicole Perlroth, Jeff Larson and Scott Shane (5 de septiembre de 2013). «N.S.A. Foils Much Internet Encryption». The New York Times. Consultado el 5 de septiembre de 2013.
