Spam filters are programs designed to filter out spam from your inbox. Once you’re on the spammers’ lists, these may be your only realistic defence against spam. Most add-on anti-spam programs use several of these methods to keep spam out of your inbox.
User defined filters
User defined filters automatically remove spam messages according to rules defined by the user. These include setting certain guidelines for acceptable sources and subject matter. For example, you can set your filter to decline all e-mail from a specific sender or all e-mail with a specific word in the header.
Header filters
Header filters examine the headers of each e-mail to determine any signs of forgery. Many spammers forge these headers to hide their locations or identities. A header contains the recipient, sender, and subject fields, as well as information about the servers that delivered the e-mail, also called the relay chain. A good header filter can detect a falsified header. However, not all spammers forge this information.
Header filters can also analyze the header of a message and compare it to a known list of spammers. If the message matches up to a blacklisted source, you won-t receive the e-mail.
Language filters
Language filters hold back any e-mail that is not written in your language of choice. If you don’t communicate with anyone in a different language, this tool could filter out unwanted messages.
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Content filters
Content filters are based on a very general set of rules that analyze the text of an e-mail to determine whether or not it’s spam. Unfortunately, these filters can hold back newsletters and other e-mail that you actually want to read.
Whitelists and blacklists
Many anti-spam programs allow you to set up whitelists of acceptable e-mail addresses. The filter will automatically allow all messages from any source on this list. Some programs offer blacklist capability as well, giving you the opportunity to block e-mail from specific sites or users.
Community blacklists
When you add on an anti-spam program to your computer, you join millions of other users in the fight against spam. Several anti-spam programs take advantage of this fact to week out spam instantly. If several trusted members of an anti-spam community mark a particular message as spam, the program will automatically block that message from other users.
Permission filters
Permission filters block all e-mail that doesn’t come from a source that you have authorized for access to your system. The permission filter is a form of challenge response system, meaning that it challenges the origin of the message and requires a response from the sender. The filter then verifies it against a predetermined list of safe sources.
Bayesian analysis
Bayesian is a learning technology that allows your system to learn what you consider spam and then use complicated algorithms to analyze incoming messages to determine whether or not they are spam. Based on words and other characteristics in the message, the Bayesian statistical filter determines a probability that the message is spam based on previous known spam versus e-mail considered acceptable. As one of a million subscribers to an anti-spam program employing Bayesian filtering, you have the benefit of the community experience and a very precise system of analysis.
This information excerpted from The Big Book of Spam Filters: Choosing the Perfect Spam Filter.
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