This Recommendation / International Standard is one of a series on message handling. The entire set provides a comprehensive blueprint for a message handling system (MHS) realized by any number of cooperating open systems. The purpose of an MHS is to enable users to exchange messages on a store-and-forward basis. A message submitted on behalf of one user, the originator, is conveyed by the message transfer system (MTS) and subsequently delivered to the agents of one or more additional users, the recipients.