There may be an instance when you've selected one of your account folders to view (such as Inbox, Drafts, Starred, Sent, etc.), but you're not seeing any email in the display:
This display will occur when the currently-selected mailbox has no messages in the chosen folder. This article covers two easy ways to fix that.
Updating your Mailbox view
If you have access to several mailboxes, you may need to change the mailbox you're viewing so that you can see messages in this folder. Use the Mailbox pull-down menu to select another mailbox, or select All mailboxes for the widest possible view:
Note: Selecting All mailboxes will show you messages from all mailboxes assigned to you.
Updating your Assigned To view
Outpost also has a menu for selecting messages assigned to specific people on your team. It's possible that you're not seeing messages in a folder because there are no messages in that folder assigned to the person you currently have selected in this menu.
Try changing the Assigned To menu to select a different user, or choose All users for the widest possible view: