1. A computer-executed method for detecting unwanted email, the method comprising: extracting a set of keywords from a received email's body; constructing a first search query using a keyword extracted from the email body; constructing a second search query using the keyword and an additional word that is a known spam topic word; issuing, to a search engine, the first search query and the second search query; receiving, from the search engine, a first number of hits and a second number of hits in response to the first and second search queries respectively; determining whether the email is unwanted, based on a comparison between the first number of hits and the second number of hits and a likelihood that the additional word corresponds to a spam topic, which involves computing a ratio between the first number and the second number; and indicating whether the email is unwanted based on the determination.