Note: To use this feature, you must be granted with Full Access to the mailbox that you wish to open.
Read the following Knowledge Base articles for more information:
How Do I Enable Send As Or Full Access Permissions On Exchange 2007/2010 Account? 
How Do I Share (Let Others See) My Outlook Folders?

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.
  3. In the Open Other Mailbox box, enter the name or email address of the mailbox that you want to open. All mailboxes that match your criteria will be displayed in a list.
  4. Select the desired mailbox and click on Open. The mailbox will be opened in a new window.

Troubleshooting:

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 to that mailbox.

Please double-check the permissions granted for this mailbox by going to HostPilot > Services > Exchange Mailboxes. Click the display name of the shared mailbox to open settings. On the Exchange Server tab, click on 'Full access & Send As' to view the list of the mailboxes with permissions.