In this paper the authors present an overview of an architectural model for large, distributed computer-based message systems. This model specifies (1) the organization of message systems in terms of functional entities; (2) the operation of the system; (3) the protocols and interfaces needed for interprocess communication; and (4) the organization of the directory system used to support identification services. General architectural considerations related to the communications protocols for computer-based message systems are presented; these follow the general framework of the ISO model for open system interconnection. The organization and operation of the directory system are discussed in detail. Special emphasis is given to the importance of identification services in international and interconnected message systems.