Microsoft To-Do is a easy to use, efficient basic planning tool. It works great for personal or small group planning. This month, Microsoft are adding the ability for users to share To-Do lists with external users on personal Microsoft account and Work or School Office 365 accounts. When it is rolled out, external sharing will be allowed by default.
While this feature is a welcome addition and helps to enrich external collaboration, in certain environments, this unmanaged external sharing can conflict with corporate policy. If this is the case, or if the risk needs to be assessed before allowing external sharing of To-Do lists, the setting can be disabled at a tenant level.
To disable the feature for your organization, navigate to the Org Settings page of the Microsoft 365 Admin Center and open the ‘Microsoft To-Do’ Pane. In here, you can disable this functionality globally. Any To-Do lists that have already been shared will have external access blocked within 24 hours after disabling this setting.
As a side note, the Org Settings page of the Admin Center contains a lot of configuration settings that people tend to miss when configuring Office 365 as these settings generally do not have a dedicated admin page to administer.