Outlook show one email unopened, but all emails are open

I am working with Outlook.  The icon for outlook in the Mac tool bar shows one message waiting to be opened.  In the list of emails, all emails are open.  I gone through all the pulldowns in Outlook and selected all and marked them all read but I still have one to open.  Is there a hidden email?  How do I get access to it so I can clear it?  Thanks.