If you set "Maximum private messages per box" to 0 (unlimited) in ACP/General/Private message settings and a user goes to UCP/Private messages/Rules, folders & settings there is an option "Move to folder:" there saying "Inbox (X out of 0 messages stored)" in the drop-down menu.

It would be better if it said just "X message stored" / "X messages stored" (or "X out of an unlimited number of messages stored" but that is a bit long for a menu item).