Why is e-mail detected as spam?

Question / Problem

Certain e-mails are sorted out as spam by SpamAssassin. Why?

Answer / Solution

The results of the integrated SpamAssassin are written in the header of the mail, just like any other SpamAssassin. If you right-click on the mail in your Outlook and select "Message Options", a dialog appears in which you can view the Internet headers of the mail.
There you will find the spam report, which gives information about which rules have been applied to this mail:
e.g.
X-Spam-Checker version: SpamAssassin 3.1.7 (2006-10-05)
     * on server1.intranet.jam-software.com
     * at Wed, 09 Apr 2008 15:56:40 +0200
X-Spam status: No, hits=1.0, required= 3.6, autolearn=no
X-Spam Report: * 0.5 HTML_40_50 BODY: Message is 40% to 50% HTML
     * 0.0 HTML_MESSAGE BODY: HTML included in message
     * 0.0 BAYES_50 BODY: Bayesian spam probability is 40 to 60%
     * [score: 0.5077]
     * 0.2 HTML_TITLE_EMPTY BODY: HTML title contains no text
     * -2.0 SP2E_BodyWordWLRule BODY: SP2E_BodyWordWLRule
     * 2.0 RCVD_IN_SORBS_DUL RBL: SORBS: sent directly from dynamic IP address
     * [87.19.116.205 listed in dnsbl.sorbs.net]
     * 0.0 RCVD_IN_PBL RBL: Received via a relay in Spamhaus PBL
     * [87.19.116.205 listed on zen.spamhaus.org]
     * 0.3 URIBL_GREY Contains to URL listed in the URIBL greylist
     * [URIs: googlepages.com]