The CCITT X.400 model of store and forward Message Handling Systems (MHS) serves as a common basis for the definition of electronic mail services and protocols both within CCITT and ISO. This paper presents an analysis of this model and its related recommendations from two perspectives. First the concepts of service, protocol and interface are discussed together with their application to this model; second the positioning within ISO's reference model for Open Systems Interconnection (OSI) is commented on.
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