Note: To use this feature, you must be granted Full Access to the mailbox that you wish to open. Full access can be granted only on Enterprise accounts.
Read the following Knowledge Base articles for more information:
How do I enable Send As or Full Access permissions on Exchange?
How do I share (let others see) my Outlook folders?

To open another user’s mailbox in Outlook Web Access:

  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 and click Open Another Mailbox.
  3. In the Open Other Mailbox box, enter the email address of the mailbox that you want to open and click Open. The mailbox will open 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 by going to HostPilot > Services > Exchange Mailboxes. Click the display name of the shared mailbox to open settings. On the Exchange Server tab, click 'Full access & Send As' to view the list of permissions.

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?