The Bayesian filter is an important part of SpamAssassin. It requires training with both spam and ham before it will operate. This training should continue on a regular basis.
Although the Bayesian filter includes the ability to automatically learn from emails it processes, this should not be the only method used to train the Bayesian filter. It is important that wrongly classified emails are not taught to the Bayesian filter, as this will decrease the effectiveness of the filter.