In the Outlook mobile app, marking an individual email as complete to show a green checkmark is not directly supported as it is on the desktop version. The desktop version allows you to mark tasks as complete using the TAGS and FLAG options, but this functionality is limited in the mobile app.
Currently, the Outlook mobile app does not provide a feature to mark emails as complete with a green checkmark. You can flag emails for follow-up, but this will show a red flag instead. If you're looking for a way to manage your tasks or emails more effectively, consider using the Outlook Web Access (OWA) on your mobile device, which might offer more functionality than the mobile app.
For now, if marking emails as complete is essential for your workflow, you may need to use the desktop version of Outlook or provide feedback to Microsoft requesting this feature in the mobile app.
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