When only SpamAssassin is in use, spam policies are relatively simple—spam tagging may be implemented either site-wide or per-user, spam may be deleted or moved to a separate folder, users may share a Bayesian database, and so on. A second filter adds an extra degree of flexibility.

In addition to simply using the results of the two filters in a filter1 AND filter2 or a filter1 OR filter2 solution, the spam policy for a site can be more complex. Email for some users might be passed through both filters, whereas mail for others may continue to be processed only by SpamAssassin, or not to be processed at all.

Even more complex policies are possible—for example, only running SpamAssassin if the score from another filter is below a certain threshold. Some filters use a simple yes/no switch and others provide a score or give a probability of the email being spam.

It is possible to implement complex policies within most MTAs. However, the simplest and most flexible method is to use...