The "Mailbox"-feature allows a normal user to upload content to the platform. This Content is visible only to him and the platform admin or owner.
Any User who has been granted the right to upload to his mailbox can use this feature.
The Admin/Owner can browse to the folder "Mailboxes" in order to see all the files any user has uploaded. From there the Admin/Owner can move the file to any location on the platform. The file is then not in the user's mailbox anymore.