To enable 2-step verification:
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Go to My Apple ID.
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Select Manage your Apple ID and sign in.
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Select Password and Security.
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Under Two-Step Verification, select Get Started and follow the onscreen instructions.
Once you've enabled 2 -step verification for your iCloud account, you have to generate an app-specific password to add your account to the Outlook app.
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Select Password and Security.
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Click Generate an App-Specific Password (a new app password is generated).
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Open the Outlook app and enter the new app password when adding your account.