Yahoo Spam filters automatically process emails/messages and filter suspected spam messages from them in order to save your PC from potentially dangerous computer viruses. However, sometimes the filter even misjudges the useful and important messages and in turn misplaced them. Therefore, users, many a time want to turn off the spam filter in Yahoo. Here’s how you can easily turn off the spam filter:

• Go to a web browser and log in to your Yahoo Mail
• After logging in, select ‘Options’ from the top-left corner and then from the drop-down menu, choose ‘Mail options’
  
• There, on the menu click on ‘Filters’ on the left in order to click on the ‘Add’ button
  
• Type ‘No Spam’ or ‘Turn off Spam Filter’ in the filter’s name text box and also type ‘@’ next to Sender Contains that move messages to the inbox.
  
• Lastly, click on ‘Save’ and make sure to double-check the saved changes.

Also read - https://writeapp.me/emailcustomerservice/note/why-is-yahoo-not-filtering-spam