As a mail hosting administrator, you can access all mailboxes from your account. Mailbox users must access their mailboxes by logging with their login information at www.inbox.com.
To to open a mailbox, follow these steps:
Log into your Inbox.com account and click on the "Manage Your Account" link.
Click on the "Domain Accounts" menu item in the side menu.
Click on the "Edit" link next to mailbox that you want to open.
On the edit form click on the "Open Mailbox" button.