When you create a search folder for all mail items in Outlook 2013 or Outlook 2016, the folder may include unexpected items with a blank subject. If you move the mouse over these items, the following information is displayed:
In folder: PersonMetadata
The PersonMetadata folder is created and used by the Outlook Customer Manager (OCM). The folder is normally hidden from other parts of the Outlook user interface. However, search folders include mail items from this folder.
Although these items have a blank subject, they are actively used by OCM and should not be deleted or modified. It is a search folder limitation to include mail items from hidden folders.