The X.400 standard, developed by the ITU-T, is a suite of recommendations that defines the architecture and protocols for Message Handling Systems (MHS). It provides a framework for a global electronic messaging service, often considered a predecessor to modern internet email. Its primary concern is the store-and-forward handling of electronic messages between users, specifying components like User Agents (UA) and Message Transfer Agents (MTA) and the protocols for their interaction. The standard ensures interoperability between different messaging systems that conform to its specifications.

B. X.500: This is a series of standards for directory services, defining how to build and access a global, distributed directory, not for handling the messages themselves.
C. X.509: This standard defines the format for public key certificates used in a Public Key Infrastructure (PKI), which is for identity verification, not message transport.
D. X.800: This standard defines the security architecture for Open Systems Interconnection (OSI), providing a general framework for security services, not a specific message handling protocol.

1. International Telecommunication Union (ITU). (1999). Recommendation ITU-T X.400 | ISO/IEC 10021-1: Information technology – Message handling systems (MHS): System and service overview. Section 1

"Scope

" states

"This Recommendation | International Standard is an overview of the MHS model

the services provided by MHS

and the protocols used in MHS."

2. International Telecommunication Union (ITU). (2019). Recommendation ITU-T X.500 | ISO/IEC 9594-1: Information technology – Open Systems Interconnection – The Directory: Overview of concepts

models and services. Section 1

"Scope

" defines the standard as providing an overview of "The Directory

" a service for looking up information about objects.

3. International Telecommunication Union (ITU). (2019). Recommendation ITU-T X.509 | ISO/IEC 9594-8: Information technology – Open Systems Interconnection – The Directory: Public-key and attribute certificate frameworks. Section 1

"Scope

" specifies that the standard defines frameworks for public-key certificates and attribute certificates.

4. International Telecommunication Union (ITU). (1991). Recommendation ITU-T X.800: Security architecture for Open Systems Interconnection for CCITT applications. Section 1

"Scope

" states

"This Recommendation defines the general security-related architectural elements which can be applied appropriately in the circumstances for which security protection is required."

5. Radicati

S. (1992). X.400 and SMTP: Battle of the E-mail Standards. Van Nostrand Reinhold. Chapter 2

"The X.400 Message Handling System

" pp. 11-13

describes the fundamental purpose and architecture of X.400 as a comprehensive system for electronic message exchange. (This is a peer-reviewed academic/technical publication).