Abstract: A system for classifying links may be used for filtering email messages and other content. Links may be classified by many methods, including analyzing registration databases and cached or actual resources referenced by the links. Using registration data, a link may be classified based on the registrar, registrant, and the date of registration. The resource referenced by the link may be analyzed using keywords as well as incoming and outgoing links to the reference. Once classified, the link may be used to classify email messages and web content for unwanted advertisement, pornography, malicious software, phishing, or other classifications.