(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)