Content Filter

A content filter is one that looks at the contents of a message and tries to guess if it is spam or a real e-mail.

Generally content filters are not vary accurate, and as they require that the mail server allow the transfer of the body of the message, they are more expensive to operate than using DNSbls.

Generally to make up for the inaccuracies in content filters, they are accompanied by a quarantine area to check for errors.

The accuracy of content filters can be greatly enhanced by using conservative DNSbls to keep the bulk of the spam out of the mail server, and then using aggressive DNSbls or fail strict rDNS checks to determine if the content filter should examine the message.

Of the content filter checks, the one that shows the most accuracy is to look up the I.P. address that any web link in the e-mail references, and check it against a DNSbl. But you only want to do that check on e-mail that fails one of the aggressive tests, or you may miss legitimate mail discussing spam and how to fight it.

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