Cannot move the task added by "Add email as task" to sub-task
Dodoist allow the structure as :
Projects ==> tasks ==> sub-tasks
However, I found that I cannot move the task added by "Add email as task" to sub-task.
You can add any email as a sub-task by using the keyboard shortcuts Ctrl+RightArrow and Ctrl+LeftArrow to indent/unintent it while you're adding it (within a project, not the "Quick Add" option).
After you've added it, you can always click on the task (or right-click and choose "Edit") and indent it while in edit mode or use the drag & drop feature. When you hover over a task, you'll see a handle icon on its left side. Grab the task by the handle and while holding it use the number keys 1, 2, 3 or 4 to set its indent level.