Sometimes the Outlook count for the Outbox folder shows it contains a message ("Outbox "), but even with all filtering off, no the list of items in the folder is empty. It looks like a message that failed to send. This happens with an account backed by a .ost file. How do I recover the missing message?
Copy the location of the .ost file from the account Data File Properties. Then close Outlook and run scanpst.exe (located in "C:\Program Files\Microsoft Office 15\root\office15" for x64 Office 2013).