(UIDL) A system used by POP3 electronic mail servers to uniquely identify a mail message. Normally, a message is identified by its position in the list of messages but this will change when an earlier message is deleted. The UIDL is a fixed string of characters which is unique to the message. The UIDL of a message never changes and will never be reused, even when the message has been deleted from the user's mailbox.

RFC 1725.