Spam Blocker identifies spam based on hundreds of characteristics. Some example characteristic are emails that begin with Dear, or emails sent with High Priority. Spam Blocker does not mark an email as spam simply because it is sent with high priority; each characteristic is weighted, producing an overall score. Spam Blocker uses this overall score to determine the probability that the email is spam. This overall score is compared to your Strength setting to determine if an email is considered spam.
Spam Blocker's default Strength (Medium) blocks most spam without interfering with legitimate email. If you increase the setting above Medium, Spam Blocker becomes more strict, thereby marking some legitimate email as spam. If you want to catch more spam than is caught with Medium strength and users don't mind sifting through quarantined email to release legitimate email, you can increase your Strength to a higher or custom setting. Keep in mind that Spam Blocker is constantly identifying new characteristics of spam, and so Spam Blocker changes its enforcement rules constantly to keep up with spammers - spam that appears in your email inbox today may just be caught tomorrow.