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Bayesian analysis

Bayesian analysis is a filter that calculates the probability of a message being spam based on the history of its contents. Unlike simple content-based filters, Bayesian spam filtering learns from spam and from good mail, resulting in a very robust, adapting and efficient anti-spam approach that, best of all, returns hardly any false positives.

Bayesian analysis is an adaptive filter - meaning it learns and makes itself better as it encounters more emails. When you first install Xeams, the Bayesian filter runs in learning mode. This is because it does not have enough data to decide if an email is junk or good. In learning mode this filter relies on other filters to assign a score to the message and updates its database.

Bayesian Repositories

The Bayesian filter manages two databases:

1. History of spam word count
2. History of good word count

These databases are stored in the config folder in Xeams and the file names are SpamWords_001.dat and HamWords_001.dat respectively. These are plain text files with two columns: Word count and the actual word.

Bayesian Graduation

To avoid extensive memory usage, the Bayesian filter stops learning once it reaches a certain word count, called as Graduation. After reaching graduation point, the filter only learns when a user specifically marks a message good or spam.

Bayesian Scoring

Bayesian scoring works in both direction - meaning if the filter determines an email to be junk, a positive score is assigned. A negative score is assigned if it is considered good.

Improving Bayesian Filter

When a message gets incorrectly tagged as good or junk, you should mark it appropriately in the message repository. Marking a message updates the Bayesian database. There are several methods for marking a message good or junk:

• By Administrator - This is done by the administrator by logging in to the admin console, viewing a message and then marking it appropriately
• By End-user - This option is off by default. When enabled, every email message sent to the end user will contain an HTML link towards the bottom of the message that can be used by the end-user to mark it either junk or good. Following steps demonstrates how to enable this filter.
• Log in to Admin Console as an administrator
• Click Action Management under the Filter Management menu
• Check Allow marking
• Specify an IP address for the Xeams server. This is the IP address of the server with respect to the end-user, which typically is the local IP address of the machine where Xeams is running. If you have users on the Internet, specify a host name that resolves to the public IP address of your network
• Modify the text for the link if needed.
• Save the changes.
  

We recommend that you run the message through the Spam Simulator to verify if Bayesian analysis will benefit from learning.  There is no point is making the Bayesian filter learn from a message if it already is determining the email's category correctly.