How does Spam Assassin decide what is junk mail?
Spam assassin searches for things like certain key words, subject heading in all caps, originating from a known spam domain,etc.. You can see the tests HERE: http://spamassassin.apache.org/tests_3_1_x.html, but be warned, it contains a lot of techie definitions, but it gives you an idea of what the Spam Assassin looks for.