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Anti-spam definition

Anti-spam is any software, hardware, or technique to protect devices and accounts from spam — the mass sending of unsolicited messages. Anti-spam tools typically focus on email security because the vast majority of online spam takes the form of junk emails. Anti-spam measures detect and weed out undesirable items like phishing emails and generic sales letters to keep the user’s inbox free from clutter.

Real anti-spam examples

  • Challenge/response systems: Senders are required to perform a task to have their message delivered. This task is typically trivial for human senders (such as completing a “CAPTCHA”,) but impossible for automated spam systems.
  • DNS disallow lists: The IP addresses of known spam sources are compiled in publicly available DNS disallow lists (also referred to as DNSBLs). Before accepting a message, email servers will query one or more such lists about the sender’s IP address and automatically reject it if it’s from a known spammer.
  • URL filtering: Many spam messages try to induce readers into clicking an embedded URL to visit a website or launch a service. Anti-spam tools can check if these URLs have been listed on publicly available domain blocklist databases — if they have, the message is marked as spam and discarded.