ISPs originally blocked messages based on only one factor, whether it was content, list quality, volume, IP address, or domain or URL blocking. Each type of filter served its purpose, but had limitations that spammers have exploited. A brief description of each filter follows:
Content filtering: Many companies and anti-spam software systems use content filtering as a method to reduce spam volume. These systems, including SpamAssassin and others, focus on the email subject line and body content to determine whether or not to deliver an email. Using point scoring, these systems focus on the email subject line and body to identify words, content, or text formatting indicative of spam. Common examples include words such as “free” or “save,” the use of ALL CAPITAL LETTERS, and the inclusion of bright colors or very large fonts. Because these are commonly used words, content filters can result in many false positive spam identifications.
List quality filtering: Many spam lists contain a large number of bogus addresses, so spammers are known to have high bounce rates at ISPs. To combat this problem, many ISPs have added list quality filters to detect when a large percentage of email addresses are bogus. If the volume of bounces exceeds a certain quantity, all of the other emails from this IP address or sending domain may be disallowed.
Volume filtering: Since many spammers send bulk emails without ...