Create a user in the Active Directory which has access to all mailboxes
Our recommendation: RdxMaildepotImporter
Start the MS Exchange System Manager
From the navigation tree, select your Exchange server, then Storage Group (for example, First Storage Group), and then Mailbox Store
Right click on Mailbox Store, then click on PROPERTIES
Open the register card SECURITY
Insert the user which you have just created and grant the user the following permissions: Send As, Receive as
Make sure that for the various options, DENY is not activated
Click on OK when you have finished
The configuration is now complete