In administrating ones email account it is too easy to accidentally delete an email and then not notice that you have deleted an important one.. Especially if you have selected a range or page of deletions.
The requested feature would allow a user to lock or unlock an individual or group of emails. A locked email cannot be deleted. If you attempt to delete a locked email you get a warning message that states that you must unlock to delete. No fancy permissions or categories are needed. . Just enough to stop you from deleting an email that you have marked as locked, will prevent most accidents.