What is Spam Assassin?
SpamAssassin is the program which is going to help you flag spam emails.
Understanding the Spam Assassin Configuration
To configure your Spam Assassin filter sensitivity, go to the relevant section of your control panel (if it applies to you). Ensure that it is enabled for the relevant user. The spam scanner will be automatically configured with a default set of settings.
Every incoming email will now be scanned. You may notice in the messages header of the email that an additional section appears like the following:
X-Spam-Checker-Version: SpamAssassin 3.1.0 (2005-09-13) on xyz.dnsvine.com
X-Spam-Level: ****************
X-Spam-Status: Yes, score=16.6 required=7.0 tests=BAYES_99, FORGED_HOTMAIL_RCVD, FORGED_RCVD_HELO, FROM_LOCAL_NOVOWEL, PREST_NON_ACCREDITED, RCVD_HELO_IP_MISMATCH, RCVD_IN_NJABL_PROXY, RCVD_NUMERIC_HELO, UNPARSEABLE_RELAY autolearn=no version=3.1.0
To understand how spam is classified, you must understand how spam is tagged. Each email is evaluated based on a number of criteria. Some of the criteria will depend on what IP address you are sending from (to determine if it is blacklisted on some Spam Blacklists), or whether or not the content appears to have certain words that are specific to spam.
Spam Assassin allocates points based on its own algorithm. The total of these points will give you the spam score for that email.
Now that you understand how the scoring works, you may be presented with some options such as "Spamassassin tag score" or "Hits required for spam" or "Spamassassin refuse score".
How this works is as follows:
Spamassassin tag score / Hits required for spam
Refers to the minimum spam score of an email before it is flagged as spam. Therefore, the lower this number, the more sensitive your filter becomes. Sensitivity ranges are from 2 to 12. Initially, you should select a setting somewhere in the middle (5-8). If you recieve a large amount of spam emails, you should make your filter slightly more sensitive (closer to 5), while if you only recive a few spams, you should set your sensitivity closer to 7 or 8. It is a good idea to frequently adjust your filter sensitivity until you find the perfect fit.
Emails that are flagged as spam will have their subject changed to be preceded by the indicator *****SPAM*****. This is just an easy way to identify which mails are flagged by the system.
Spamassassin refuse score
Some systems provide this option, which will tell the server to discard any emails that come in above the score stated here. As with the tag score above, the lower this number, the more sensitive the filter becomes. This score has to be above the Spamassassin tag score.
On Plesk based servers, this option does not exist. Instead, an option is provided to delete all emails that are flagged as spam by the "Hits required for spam" setting.
If you find that a number of legitimate emails are being mistaken for spam, you may want to reduce your filter's sensitivity level. If you notice that a number of spam emails are making it through to your inbox, then increasing your filter's sensitivity should help.
Once you have completed these steps, Spam Assassin will effectively filter at least 95% of the spam emails sent to your domain.