Symptoms
You notice that SpamBox is marking messages as spam that should not be considered spam.
Description
This can sometimes occur if there is corruption in the SpamAssassin Bayesian database.
Workaround
Move aside the bayes_ files from the /home/$user/.spamassassin directory. SpamAssassin will automatically re-create these files and will start using the Bayes database in scoring again once the number of emails in the database has reached 200.
See also this page on how to teach SpamAssassin to identify spam: