Unique ID Listing
<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.
RFC 1725 (http://ds0.internic.net/rfc/rfc1725.txt).
