(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)