This article will show you how to access additional mailboxes you have been granted permissions to in Web Mail.
If you are using the Light Version of Outlook Web Mail follow the below steps:
You will need to enter the email address of the mailbox in the address bar of your web browser and press "Enter" (or "Return").
If you receive the error "You do not have permission to open this mailbox. For access or for more information, contact technical support for your organization", you don't have access to the mailbox in question.