By around 1980, it became clear that email systems within an organization would need to be able to exchange information with a much larger set of  external people and organizations. It was thus thought that a way was needed to let people browse external address books, so as to identify someone's email address, and then send them email. A standard way of connecting email address books was defined, and called X.500.
The idea was inspired directly by traditional telephone books. Everyone has a directory of phone numbers of local people: probably in their organization, or in their town. To find the phone number of someone more distant, you contact a directory service which then locates the directory that applies to the person you want to telephone. The service then retrieves the telephone number for that person and gives it to you. You can then call them.
The material in the gallery describes the substantial efforts of computer vendors and IT professionals to develop and implement this attractive vision. It turned out to be all for naught. Much like X.400, the X.500 standard didn’t catch on.
Unlike X.400, x.500 was not superceded by an alternative.  By about 2005, the attempt to provide a homogeneous directory system was abandoned, and it's now largely forgotten. As of 2012, we still get by without a global email address lookup system, even 'though it's inconvenient at times.-–David Ferris & Nigel Dutt