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Sign in to your account in Outlook Web App. (outlook.office365.com)
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On the Outlook Web App navigation bar, select your name. A list appears.
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Select Open another mailbox.
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Type the email address of the other mailbox that you want to open and then select Open. Or, start typing and then select Search contacts and directory to find the mailbox you want to open. Select the shared mailbox you want to open, and then select Open. Another Outlook Web App session opens in a separate window, allowing access to the other mailbox.