While Gmail doesn't filters all the spam messages that could reach your inbox, it certainly does a better job than other webmail apps like Yahoo Mail or Hotmail. 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. But spam is also evolving and it's harder to block, especially when it uses images and literary texts.
"Many Google teams provide pieces of the spam-protection puzzle, from distributed computing to language detection. For example, we use optical character recognition (OCR) developed by the Google Book Search team to protect Gmail users from image spam. And machine-learning algorithms developed to merge and rank large sets of Google search results allow us to combine hundreds of factors to classify spam," explains Google.
"Gmail supports multiple authentication systems, including SPF (Sender Policy Framework), DomainKeys, and DKIM (DomainKeys Identified Mail), so we can be more certain that your mail is from who it says it's from. Also, unlike many other providers that automatically let through all mail from certain senders, making it possible for their messages to bypass spam filters, Gmail puts all senders through the same rigorous checks."
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