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.
Dangers lurk in one user masquerading as another.
For example, no facility ensures that a user does not impersonate and misuse a Message Transfer System or falsely claim to originate a message. No facility ensures that message sequence is preserved. If a message were resequenced, messages could be replayed, reordered, or delayed. With the products based on the 1988 specification emerging, X.400 can serve a more useful role.