just to give an idea of the way SpamAssasin reports the spam scores. SpamAssasin is a very common blocker.

mail above the SPAM threshold

Subject: [SPAM] A bigger, bett result with Cialis
Content-Type: multipart/alternative;
        boundary="--155414326368415972"
X-Spam-Flag: YES
X-Spam-Checker-Version: SpamAssassin 2.63 (2004-01-11) on maxio7.uk2net.com
X-Spam-Report:
        * 2.4 ONLINE_PHARMACY BODY: Online Pharmacy
        * 0.1 HTML_FONTCOLOR_GREEN BODY: HTML font color is green
        * 0.2 HTML_50_60 BODY: Message is 50% to 60% HTML
        * 0.1 MIME_HTML_ONLY BODY: Message only has text/html MIME parts
        * 1.1 HTML_MESSAGE BODY: HTML included in message
        * 0.8 PRIORITY_NO_NAME Message has priority setting, but no X-Mailer
        * 1.1 MIME_HTML_ONLY_MULTI Multipart message only has text/html MIME parts
X-Spam-Status: Yes, hits=5.8 required=5.0 tests=HTML_50_60,
        HTML_FONTCOLOR_GREEN,HTML_MESSAGE,MIME_HTML_ONLY,MIME_HTML_ONLY_MULTI,
        ONLINE_PHARMACY,PRIORITY_NO_NAME autolearn=no version=2.63
X-Spam-Level: *****
X-SA-Exim-Version: 3.1 (built Mon Jan 5 23:23:23 CET 2004)

Heather's recent mail shot. It is HTML with a graphic and links. If I set the spam threshold lower it would have been labelled as spam.

X-Spam-Checker-Version: SpamAssassin 2.63 (2004-01-11) on maxio9.uk2net.com
X-Spam-Status: No, hits=2.2 required=5.0 tests=HTML_FONTCOLOR_BLUE,
        HTML_FONTCOLOR_RED,HTML_FONT_BIG,HTML_MESSAGE,HTML_TAG_BALANCE_BODY,
        NO_REAL_NAME,UPPERCASE_25_50 autolearn=no version=2.63
X-Spam-Level: **