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Answer: Here are a few basics:
Distributed content filters. Several anti-spam companies help ISPs and enterprise customers cope with the influx of unsolicited email. Brightmail and Postini are two leaders in this field. These blocking systems employ complex content analysis heuristics that scan message content and create message "signatures" that are disseminated among the filtering company's client base.
ISP content filters. Similar to distributed content filters, ISPs often employ content filters created internally or adapted from others. Content filters scan for a variety of red flags. They can even learn new patterns in spam email, such as inserting periods in words that would normally trigger a block. SpamAssassin is a popular open-source content filter and one of the best examples of how these filters operate.
User content filters. Almost every email client provides junk mail filters. These vary widely in complexity. Microsoft Outlook's filter simply searches for offensive keywords and key phrases, whereas more robust filters can be configured to run from a user's desktop.
In general, content filters look for words, phrases and symbols that are often used by spammers. Secondly, and perhaps less known, is filtering based on improper HTML coding such as one used by AOL. See question/answer # 4 and #6 for more information.
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