Does Your Website Host Fight Spam? - Keywords and Regular...
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Keywords and Regular Expressions
More advanced server level Spam filters are available. A small advance is accomplished using keyword filters. Keyword filters merely check for instances of a certain string of characters and deny the message if that string if found. The core problem with keyword-only filters is they can "over filter". Someone who puts "sex" on their keyword filter will find receiving local news and event announcements difficult if they live in a town named "Essex". Some filters attempt to address this deficiency by using "regular expressions" in order to build a sophisticated rule set to prevent Spam from reaching your inbox. Briefly, regular expressions are syntax rules used to identify certain strings of text or numbers. These rules can be set up to identify text patterns that are commonly used in Spam. They can become quite complex, but, as with most any filtering method, are not 100% bullet proof. Some filters that use regular expressions come with a basic set that can be appended by the user. Obviously this kind of feature is of little use to someone not familiar with regular expressions.
Bayesian Filters
Currently the most sophisticated filtering methods use Bayesian inferences. Bayesian filters take a large data set and determine the probability a message is Spam based on its similarity to previous Spam messages. The more emails that are processed and flagged theoretically make the filter more accurate. Services that provide filtering on an ISP or host level, like Postini's "SpamAway", filter billions of emails and provide the highest level of success and fewest "false positives".
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