(UIDL) A system used by POP3electronic mailservers 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 .(1997-04-16)