Messages sent from these addresses receive a detrimental score

Messages from addresses on this black list will have their Spam Filter score increased by the amount specified on the Spam Filter Settings screen, typically causing them to be marked as spam. However, including an address on this list does not automatically guarantee that a message from that address will always be considered spam. For example, if a message comes from a black listed sender but is addressed to a white listed recipient, then the score modifiers may offset each other and cause the message to have a final score that is below the spam score threshold. This could also happen if you have the black list score modifier set particularly low.