SpamAssassin is run on every email message that is delivered to a CS email account. It is not run on messages that are forwarded to other email addresses via system mail aliases or .forward files.
SpamAssassin gives each email a score representing the degree to which SpamAssassin believes the email is spam. Negative numbers indicate a strong likelihood that the message is legitimate (i.e. not spam). The default threshold for treating an email as spam is a score of 7, but you can change that if you want.
When SpamAssassin decides that a message is spam, it does four things:
- It adds "**SPAM**" to the email's subject line. This gives a quick visual indication that it's been classified as spam.
- It adds an X-Spam-Flag: YES header. This makes automated message parsing easier.
- It adds an X-Spam-Status header, which gives an overview of SpamAssassin's scoring decisions for the message.
- It adds an X-Spam-Report header, which gives a detailed listing of each scoring rule that SpamAssassin applied to the message.
Although SpamAssassin gives every email a spam score, it is up to you to do something with that score, so SpamAssassin has no effect unless you filter your mail with Procmail. For information about that, please see Filtering Spam with Procmail.
You can also change SpamAssassin's configuration, which allows you to customize things like its threshold score for classifying a message as spam, its treatment of spam emails, and the details of its scoring decisions. For more information about that, please see SpamAssassin Configuration.