Content filtering is the most commonly used group of methods to filter spam. Content filters act either on the content, the information contained in the mail body, or on the mail headers (like “Subject:”) to either classify, accept or reject a mail.
The most popular filter is the Bayesian filter, which is a statistical filter.
Usually Anti-Virus methods can be classified as content filters too, since they scan (simplyfied) either the binary attachments of mail or the HTML contents.
Common content filters are:
Bayesian
Attachment
Mail header
Mailing List
HTML anomalies
Language
Heuristic
Regular Expression
Phrases
Proximity
URL
Content-encoding
Char-set
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