Source : Free On-Line Dictionary of Computing

Unique ID Listing
     
         (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)}.
     
        (1997-04-16)