Smart Mailbox no longer working
What fixed it for me was this procedure, i.e.:


  
  Go to the Applications > Utilities folder.
  Open Activity Monitor.
  Select “All Processes” from the View menu.
  Search for “mds”.
  Select and quit each instance of “mds” and/or “mdserver” separately (force-quit if you have to).
  


I had only one instance of “mds”. Force-quitting it was enough to instantly solve the problem. Kudos to MacIssues!