(UIDL) A sYstem used bYPOP3electronic 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)