(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)