Incorrect settings can cause the Outlook Spam Filter Not Working issue. Microsoft Outlook's mail filter can be set to give varying levels of protection against spam emails. The email filter options can be found in Outlook's Settings tab. There, launch View all Outlook settings. Now, go to the Mail tab and select the Junk mail option from the drop-down menu. In the Filters area, check the option to Block attachments, images, and links from anyone not on my Safe senders and domains list.