Spamassassin is the Anti-Spam software used by most Linux based email systems. Spamassassin is a complex system that is modular and allows for the use of custom tests and score adjustment. Each email sent to SpamAssassin for checking goes thru a large number of individual tests. Each check may add or subtract to an email’s score. As email gets tested, each test score is added to a running total.
When this total reaches the Spam Threshold as set by the Administrator or the User, the email is marked as spam. The standard score is set at 5.0. The higher you set this, the more “spammy” email is allowed through and the lower this is set, the more chances there are that valid email gets blocked.
SpamAssassin itself never blocks email. It only makes a recommendation and gives a score. Your server’s Mail Transport Agent is then left to decide what to do. Often, spam is specially marked with “***SPAM***” in the subject. This allows you to filter these emails in Outlook and never risk losing an email that should not have been marked as spam (false positive). Another approach is to push all “Spam” into a Spambox – a special mailbox just for spam. This mailbox is usually in the end user’s email folders and also allows the end-user full control.
There are a lot of free tools out there that you can use to check to see your email’s spam assassin rating.