Spamassassin
Out of the box, SpamAssassin does a reasonably good job of tagging
spam, however, you can make it even better by enabling bayesian filtering.
Unfortunately, the increased effectiveness doesn't come for free, you will
need to actively train SpamAssassin's bayesian filter by telling it
about spam messages it missed.
System Default Filtering
The majority of CS accounts are using our system default bayesian filtering.
By default it does a good job filtering out spam, but you can help by
bouncing any spam that does get through to "spamreport" or any good mail
that ends up in your spam to "hamreport". How you can do this is explained
here.
