When your Outlook mail account is hosted on an Exchange server, the server may periodically remove older messages from your regular mailbox and store them in a separate archive mailbox.

Information provided in these steps applies to Microsoft Outlook Web App for Office 365 and Exchange Server 2013. Instructions may vary slightly or significantly for other versions of Outlook.

Microsoft also added a new protocol for Exchange and Outlook communications called "MAPI over HTTP," which can be used by Outlook 2013 SP1 clients, replacing the Outlook Anywhere protocol.