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X.400

A messaging standard that can be used by many messaging systems. X.400 uses a strict addressing method that reflects a hierarchical environment. An X.400 address reflects the recipient’s position in a messaging hierarchy.

X.400 Connector

A connector that can be used to connect an Exchange system to a foreign X.400 system or to connect two Exchange routing groups over unstable or low-bandwidth (typically less than 16 Kbps) connections.

X.500

A standard that defines how directories should be structured. X.500 directories are hierarchical, with different levels for each category of information. Both Active Directory and LDAP are loosely based on the X.500 standard.

X.509

A standard that defines digital certificates.