Here the very first things to check here would be your internet connection and the second is to confirm Outlook is not set to “Work Offline”. To find this information open outlook and at the bottom right-hand corner, there is a taskbar that shows the connectivity status. You want this to say, “Connected to: Microsoft Exchange”. If the application is in offline mode it will say so here.
To disable offline mode: Open outlook, navigate to the Send / Receive tab at the top left-hand corner. Look for a tab that says “Preferences” and click on the Work offline icon.
To force sync your mailbox: Navigate to the Send / Receive tab on the top left-hand corner. Select the Send / Receive all folders. To sync just the one folder you have selected you would select “Update folder”
Another thing to check is if the application requires Credentials to be entered. To confirm this, look at the bottom taskbar next to “Connected to:” if your application requires your credentials it will not allow new emails to be received or sent out.
Another basic troubleshoot would be to close the application completely and reopen.