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Configuring Spam Filters

The amount of unsolicited commercial email ("UCE" aka "Spam") is always growing. Although our users are asking us to suppress these, German law does not allow us to do so.

We therefore offer the following procedure:

  • All incoming email is automatically checked for spam characteristics by the program Spamassassin on a central server. Points for every spam check are counted and the total score is written to an additional header line. Otherwise unchanged the mail is then delivered.

     

  • You may enable a filter in your mail client that examines the spam score and takes special measures.
    • If you do nothing everythings remains the same.
    • You may direct mail with a score above a certain threshold to a special folder. In the beginning you should examine this folder from time to time; it might happen that some important email gets a high spam score.
    • You may delete mail with a score above a certain threshold when you have found your optimal threshold and when you are aware that the scoring is not in 100% correct so you might lose important mail.
    • Of course, you may configure several filters (in the right order). For instance such that mail with a score above 15 is deleted immediately and mail with a score above 8 is directed to a special folder.
  • So you are in full control of your mail.

Your mail client will usually not show you the additional header lines. If you want to see what happens you can turn them on (see the configuration examples below).
The header lines look like this:

Subject: Sweet Jeanne: "Opps! There goes my Shirt!"
X-Spam-Status: No, hits=11.9 tagged_above=1.0 required=99.0
 tests=ALL_CAP_PORN, CLICK_BELOW, CLICK_BELOW_CAPS, CUM_SHOT,
 FOR_INSTANT_ACCESS, FREE_MEMBERSHIP, HOT_NASTY, INSTANT_ACCESS,
 MAILTO_TO_REMOVE, MANY_EXCLAMATIONS, MISSING_HEADERS, NO_REAL_NAME,
 SPAM_PHRASE_08_13, UPPERCASE_25_50
X-Spam-Level: ************

In X-Spam-Status you find the score (here 11.9 points).
[No, tagged_above and required are not relevant here.]
tests lists some hints on how the score came about.
X-Spam-Level shows the score again as a string of asteriscs, because this representation is more easily parsed by mail clients.

Filter Configuration in Mail Clients:

(Sorry, currently German only, but the screenshots may be useful)

 

 

The Choice of the Theshold:

To get an idea for the choice of the threshold I have had scored 1615 mails containing spam (July to November 2002). Spam score plotted to the right:

Spam0

For comparison the histogram of a mail folder (almost) free of spam containing 2383 emails:

No Spam

Obviously it is not useful to configure a threshold below 5 if you do not want to lose "real" emails.
Please note that these histograms are typical for my mail mix. It may look different for your's, e.g. if you receive many mails in other languages.

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