With the last update of To-Do across platforms we have enabled the option to see flagged emails from Outlook as Tasks in Microsoft To-Do. Simply flag an email in Outlook to create a task from it in To-Do.
You can enable this feature in Microsoft To-Do from within the App Settings.
To get the latest version of To-Do please go to https://products.office.com/en-us/microsoft-to-do-list-app
An error occurred while saving the commentMartin commented
The offering to show Flagged e-mail is potentially useful, but only shows the messages flagged in my outlook.com account in Outlook 365, not in the other e-mail accounts. This restriction is not stated.