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X.400
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An international message-handling standard for connecting e-mail networks and for connecting users to e-mail networks. X.400 is published by the X.400 Application Programming Interface Association. MAPI applications are fully compatible with X.400 messaging applications.
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X.500
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An international message-handling standard for directory services, published by the International Telegraph and Telephone Consultative Committee (CCITT) standards body.
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