Missing Outlook Emails
Outlook has recently introduced a feature that adds a "Focused" section to your regular inbox. This can cause emails to go "missing" when they are just under a different view. You can choose to continue using this new way, or disable it by following the below steps.
Desktop Application
1. Click the "View" button on the top toolbar, then click "Show Focused Inbox" - this will put all emails into the "All" tab by default.
Web Application
1. Click on the settings gear in the upper-right, then go down to "Focused Inbox" and click the button to turn it off.