There are a variety of reasons for your Yahoo Spam Filter Not Working properly. If you build a filter with no rules (i.e., an empty filter), it will send any messages to the inbox that contain an empty character. When this filter is applied on top of other filters, it nullifies their rules and sends all emails to the inbox. Alternatively, if the rules are incompatible, you'll encounter this problem. The use of both positive and negative terms together is an example of conflicting norms.