The Message Store, always connected to the Message Transfer System, stores messages for the User Agents. User Agents submit messages through the Message Store as well as retrieve, list, summarize, and delete messages from the Message Store database. P7 support is crucial for anyone using laptops. The 1988 X.400 recommendations suggest using ISO's X.500 directory service for naming, storing distribution lists, storing profiles of User Agents, and user authentication. By using X.500, users do not have to contend with ungainly machine-oriented names, but can use more intuitive names.
Pilot projects are under way, but there are no commercial implementations.
No clear-cut solution exists for a directory service that's suitable for a heterogeneous network. Security is another key issue. The 1988 spec provides facilities to authenticate who originated the message, verify who originated a delivery or nondelivery notice, check that the message or its contents were not altered and that all recipients received a copy. It also provides return receipt and registered mail services. Security must be improved beyond these capabilities.