A new feature is currently being deployed on Exchange Online (first to Targeted Release organization) to simplify your task management.
This new feature will be/is only available when using Outlook Web Access (or Outlook on the Web as they call it now) and when having turn on the option Try the new Outlook
The new Tasks pane is available directly from your Mail or Calendar section and allows you to drag and drop email to automatically create a new task.
You can show the Task pane you need to click on the “ToDo” icon shown on top right, left side of the gear menu
And as the Tasks in your mailbox are synced with ToDo, this made these new tasks also available on your ToDo mobile app