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Spam filters, the focus of this guide, are software programs used to detect unsolicited and unwanted email and preventing these messages from getting to a user's inbox.

Most email spam messages are commercial, and most recipients find them to be annoying, intrusive, and potentially dangerous. Many people find that as much as ninety percent, or more, of the email they receive, is spam. It can be difficult to pick the few wanted messages from the many unwanted ones, meaning that important emails are sometimes missed or deleted as spam. Spam can also be dangerous because they may contain links that lead to phishing sites or sites that host malware, or they may include malware or viruses as attachments.

Spammers collect email addresses from several places, including social media, websites, customer listings, and viruses that harvest user address books. Collected email addresses are often sold to other spammers.

A spam filter is a software program designed to detect unsolicited and unwanted email and to prevent these spam messages from getting to the user's inbox. Like other filtering programs, a spam filter uses certain criteria upon which to base its judgments.

Simple filters might look for specific words or phrases in the subject line of email messages, then excluding these from the user's inbox. However, spammers quickly learn to use different words and phrases, while legitimate messages may be blocked. Other types of spam filters, such as the Bayesian filters, Markovian discrimination, or other heuristic filters, differentiate spam from legitimate messages by suspicious word patterns or word frequency.

Other methods include whitelisting, in which identified sender IP addresses, email addresses, or domain names are passed through. Blacklisting may also be used. Also known as blocklisting, this is a method that specifically excludes certain email addresses, IP addresses, or domain names. In most cases, blacklisting makes use of public anti-spam lists that are built through the experiences of other users.

Many, and perhaps most spam filters employ multiple methods, some of them optional. Whatever the method clients designed to filter spam from email are the focus of topics in this category.



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