The Unread Mail folder is one of Outlook's default Search Folders. If deleted, you can create it again.
In Mail, on the File menu, point to New, and then click Search Folder.
Tip: You can also open the New Search Folder dialog box by pressing CTRL+SHIFT+P.
In the Reading Mail section, click Unread Mail.
Note: This procedure works for restoring any default search folder; choose that folder in the New Search Folder dialog box.