X.400 addresses

X.400 email addresses have many possible entries mostly optional.
The values generally used are:
Country
the country must be specified
ADMD
the identity of the X.400 service provider
PRMD
the top level of the organization using X.400
Organization and organization units can be used to subdivide the company into logical groupings. The user is identified using some combination of first name last name initials and nickname.

X.500 Directory Standard
X.500 is a standard that is applied to directory services rather than email systems.
An email system can implement X.500 within its recipient directory and can then integrate it with other X.500 compliant directories.
X.500 directories can be organized into a single global directory than can offer thousands of names for querying on the Internet.
X.500 allows data to be organized in a hierarchical system in which users can be located by organization department or name.
One of the most popular X.500 directory services is Novell NetWare Directory Services (NDS).