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Select File > Info > Account Settings > Account Settings.
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On the Email tab, choose your email account and select Change.
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Select More Settings > Advanced > Add. Enter the email address of the shared mailbox and select OK and OK again.
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Select Next > Done > Close.
For more info, see Open and use a shared mailbox in Outlook.
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In Mail, in the left folder pane, right-click your account name, then select Add shared folder or mailbox.
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In the Add shared folder or mailbox dialog, type the name or email address of someone who has shared a mailbox with you, and then select Add.
The shared mailbox appears in your Folder list on the left. To remove it, you can right-click the shared mailbox, and then select Remove shared folder.
For more info, see Open and use a shared mailbox in Outlook.