messaging (UIDL) A system used by POP3 electronic mail

servers 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.