How to Add a Second Mailbox in Outlook (Microsoft 365)

For Outlook Desktop (Windows):

Open Outlook.

Go to File > Account Settings > Account Settings.

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Under the Email tab, click New.

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Enter the email address of the second account and click Connect.

Enter the password when prompted and complete the sign-in process.

Once added you need to restart Outlook, the second mailbox will appear in the left-hand folder pane (it might take some time for everything to sync).