Spamassassin rules, or why my important e-mails get stuck in my anti-spam?

When using spamassassin, you might happen to realize that some important e-mail has been filtered out for some kind of wierd reason. The e-mail will be classified as spam and get an introduction like this:
Content analysis details: (32.9 points, 5.0 required) pts rule name description ---- ---------------------- -------------------------------------------------- 3.5 BAYES_99 BODY: Bayesian spam probability is 99 to 100% [score: 1.0000] 0.4 SARE_SUB_ENC_ISO2022JP Subject specifies display in non-English lang 1.0 GAPPY_SU