Working with backup sets : Email message backup sets (Windows) : Before you begin : Microsoft Outlook requirements (DS-MLR)
 
Microsoft Outlook requirements (DS-MLR)
Ensure that the following requirements are met wherever applicable before you create an Email messages backup set to back up Microsoft Outlook data using DS-MLR:
Ensure that Microsoft Outlook is closed during backup activities. Backup is successful only when Microsoft Outlook is closed.
If you have configured Microsoft Outlook to use a .PST file on a remote machine, the DS-MLR service must always be installed on the computer with Microsoft Outlook.
For Microsoft Outlook configured for Microsoft Exchange profiles, the email messages are located on the Microsoft Exchange Server. To access those email messages, the DS-MLR service account must have the Receive-As privilege on that Microsoft Exchange Server. The DS-MLR service account must be recognized on the Microsoft Exchange Server. Otherwise, DS-MLR will return an access denied error.