Unread Mail folder is an Outlook default folder that quickly filters and store all unread emails to display them further. It is located under the search folder area and also shown as a favorite folder for user convenience.
Basically, "Search Folder" contains an Unread Email Folder, For Follow Up, Large Mail, Categorized mail folders as a sub-folder. But what if you mistakenly deleted the Unread Mail Folder after finding it useful anymore.
Don't worry, deleting a search folder does not really delete emails which have not read till yet. The emails are still available in their original folders. However, this folder provides a direct and simple approach to view unread emails.
There is a different path for different Outlook version to enable Unread Mail Folder.
File >> New >> Search Folder
Tip: New Search Folder dialog box also can open by pressing "CTRL+SHIFT+P".
Note: This process work for the restoring of default search folder, select folder in New Search Folder dialog box.
Now the "Unread Folder" appears in the "Favorites" section of MS Outlook.