Mail maintains an internal database of information that helps it detect junk mail. When you mark messages as junk or not junk, Mail updates the junk mail database accordingly and your junk mail filter continually improves over time. If you change your mind about what is junk mail (for example, you want to receive someones messages that you previously specified were junk messages), you need to mark them as not junk.
Mail continues to update its junk mail database when you mark messages as junk or not junk, regardless of whether youre currently using the database in your filter. To reset the junk mail database to its original information, and remove everything Mail learned from you about what is junk or not junk, click Reset.