Bayesian, Bayes, and naive Bayes are often used in discussion of POPFile and similar email filters which usually refers to a mathematical formula.

In the 1700's Thomas Bayes was a man who studied probability, his work has since become known as Bayesian Statistics. And this method has recently become popular in email filtering because it does a very good job determining between different groups of messages.

Bayes Theorem gives POPFile a way to calculate the probability that an email is work, personal or spam by calculating P(work|E), P(personal|E), and P(spam|E) where E is the new email and P(work|E) is the probability of email E being a 'work' email and so on. By picking the largest probability of the three POPFile can automatically pick the appropriate folder. POPFile calculates these probabilities by looking at the frequency with which words occur in each folder and applying Bayes Theorem.

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