unique id listing

from The Free On-line Dictionary of Computing (8 July 2008)

Unique ID Listing
UIDL

   <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).

   (1997-04-16)