Symptom

A folder in Outlook appears to have no messages and you know there are messages in that folder.

Solution

Microsoft Outlook has the ability to set up customized views for items such as email located in folders. Sometimes, particularily after a large import from another account such as Gmail or an IMAP-based account, the custom view settings hide the emails from view. The solution is to reset the view for the folder or the entire mail box.

Reset a View on a Specific Folder Outlook 2010 and Up

A Reset button is also on the View Settings dialog (formerly the Current view dialog, below), which can be opened from the View ribbon, View Settings button or by right clicking on the field names visible in a view and selecting View Settings (Method 3, below).

You can expand the Change View menu and select Manage Views (formerly the Define Views dialog, below), then select the View by name to reset the view globally or select <Current view settings> to reset the view on the current folder only.

Create new views, edit views, or delete custom views using the Manage Views dialog.

Reset All Views in All Folders

Reset the views for the entire mailbox. When you open Outlook using the /cleanviews switch you will wipe out all custom views and return all default views to "factory default". Sometimes its necessary, but sometimes just resetting the view for the current folder will work. If you aren't using custom views, you can use the /cleanviews switch without worry.

The /cleanviews switch will wipe out all custom views and return all default views to "factory default". 

To use this method, close Outlook. Go to Start menu > Run  (or Windows key + R) and type Outlook.exe /cleanviews then click OK.