Outlook 2003 Junk-Mail Filter Run Amok

No matter what settings I enter for the Outlook 2003 junk-mail filter, all messages downloaded from my ISP end up in the junk-mail folder. I then have to click on each one and indicate that the sender is safe in order to move the message into my in-box. Even after having done this, future messages from the same sender end up in the junk-mail folder.

Neither the Outlook 2003 help files nor the Microsoft Knowledge Base offer any clues on the existence of such a problem–let alone a solution.

I’m running XP, SP2, and updates from Microsoft are current. Any suggestions? Thanks!