In order to be able to open another user's mailbox, you need to give your account Full Mailbox Access to that mailbox, which you can do using both the Exchange Management Shell (Add-MailboxPermission) or using the Full Mailbox Access wizard available in the Exchange Management Console since Service Pack 1. If you have a user though that has been given Full Mailbox Access on a higher level (server-level, storage-group-level, or higher), you will notice that the user is able to open mailboxes in that storage group using Outlook, but when trying to open the mailbox using Outlook Web Access the following error message appears: You do not have permissions to open this Mailbox. Contact Exchange Administrator for more information.In order to be able to open another user's mailbox, you need to be given Full Mailbox Access on the mailbox itself. Because only the mailbox security descriptor is verified when the underlying components are shared by Outlook Web Access and by Exchange Web Services. For more information, please refer to: Error message when an account tries to open a mailbox by using Outlook Web Access or Exchange Web Services in Exchange Server 2007: "You do not have permissions to open this mailbox"