Show AllSearch Folders are a way to gather e-mail messages and other Microsoft Outlook items into a folder based on search criteria. For example, you want to see all messages that you have not yet read. The messages can be viewed from but are not stored in Search Folders. Search Folders are virtual folders that contain views of messages that might be in the Inbox and other folders you have created.
In All Mail Folders, in the Navigation Pane, there are three default Search Folders — For Follow Up, Large Mail, and Unread Mail. Any e-mail item with a flag appears in the For Follow Up Search Folder. E-mail items that are larger than 100 kilobytes (KB) appear in the Large Mail Search Folder. All unread e-mail items appear in the Unread Mail Search Folder.