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Follow the steps to add a shared mailbox in Outlook 365
Click the File tab in the top left corner.
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Click Account Settings and then Account Name and Sync Settings
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Click on More Settings
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Go to the Advanced tab and press Add…
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Enter the name of the mailbox and press ok
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Remove the check mark from “Download shared folders”
Press Apply and
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restart Outlook.
The mailbox will now appear in the left pane under your personal mail.