To use this feature, you must be granted Full Access permissions for the mailbox that you wish to open. Read the Knowledge Base article on How Do I Share (Let Others See) My Outlook Folders? for more information.
To open another user’s mailbox:
1. Log in to OWA using your own mailbox email address and password.
2. Click on your name in the upper right corner of the window, as shown in the following screenshot.
3. In the Open Other Mailbox window, enter the name or email address of the mailbox that you want to open.
4. If more than one matching entry is found, Outlook Web Access will list them all.
5. Choose the mailbox to open. The mailbox will open in a new window.
If the mailbox doesn’t open and you get the following error message instead:
"You do not have permission to open this mailbox. Contact technical support for your organization for access or for more information."
then you need to make sure that you were granted Full Access permissions for that mailbox.
In case you need to open mailbox in Outlook client after full access is given please read the Knowledge Base article on How Do I Add Someone Else's Mailbox To My Exchange Outlook Profile?