An international standard (1988) developed jointly by the International Organization for Standardization (ISO) and the ITU-T to support the e-mail name and address lookup requirements of X.400. X.500 provides for global directory services that theoretically enable network managers to store information about all users, machines, and applications in a distributed fashion. X.500 is a very robust global directory standard that requires significant computational resources to implement and, therefore, is criticized for being over-engineered. See also e-mail, ISO, ITU-T, LDAP, X.400, and X Series.