UIDL
       Unique ID Listing (POP3, RFC 1939)
       

Unique ID Listing
UIDL

    (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>.

   (1997-04-16)