X.400 is a suite of protocols defining standards for email exchange among networks. Developed by the International Telecommunication Union Telecommunication Standardization Sector (ITU-T), it’s part of the X.4000 series that defines the email framework at the Open Systems Interconnection application layer. It supports global electronic messaging, including interconnection with other email systems like SMTP.


The phonetics of the keyword “X.400” would be: “eks. four zero zero”

Key Takeaways

  1. X.400 is a set of communications protocols that defines messaging services for electronic mail in the OSI (Open Systems Interconnection) model.
  2. Unlike other email protocols such as SMTP, X.400 includes comprehensive features that offer advanced functions like message recall, report upon message delivery or read, and security capabilities such as authentication and privacy protection.
  3. X.400 is not as widely used as SMTP for email exchange over the internet, largely due to its complexity and the cost associated with its implementation and maintenance.


X.400 is important in the realm of technology as it refers to a suite of standards, developed by the International Telecommunication Union, defining protocols for global email transmission. Established in the 1980s, it was notable for its capacity to provide uniform messaging standards in an era where consistency was lacking. X.400 set the groundwork for facilitating communication amongst different systems, including defining address conventions and message content protocol. Though it’s been somewhat superseded by the Simple Mail Transfer Protocol (SMTP), X.400 remains vital in environments where complex addressing is required, such as military and government operations, demonstrating its relevance and importance in the field of electronic communication.


X.400 is a suite of protocols defined by the International Telecommunication Union (ITU-T) for global electronic messaging, particularly email communications. X.400’s main purpose is to facilitate the global transmission of electronic messages, specifically between different computer networks and systems, enabling consistent and reliable communication. Although developed in the 1980s, X.400 remains the benchmark for modern email standards due to its robust features that include security provisions like authentication and confidentiality.The primary use of X.400 is its establishing of standards for messaging services. What this means is that it permits the seamless exchange of messages across different vendors, countries, organizations, and a range of diverse computer systems. For instance, a user can send mail from a Unix-based system to a receiver on the Microsoft Exchange server without any compatibility issues. It also supports various types of messages not just text, including audio, video, and image files, among others. Integrated directory services within X.400 further help in addressing and routing messages to the correct recipients.


1. Microsoft Exchange Server: X.400 is used in Microsoft Exchange Server as a part of a communication protocol. Microsoft Exchange Server allows businesses to securely send and receive emails and other forms of interactive communication through computer networks. X.400 ensures that this is done in a structured, reliable, and efficient format.2. Government Communications: Many government organizations all around the world utilize X.400 for their communication needs. For instance, in the 1990s, the British government used X.400 for its “Government Secure Intranet.” X.400 was chosen because it was a proven standard that offered both security and reliability.3. Airbus: The aeronautics manufacturer Airbus uses X.400 in their communication processes. Considering the international nature of its operations, Airbus needed a messaging protocol that would be widely accepted and understood by different systems – X.400 provided this standard. It allows for the efficient exchange of various types of documents and communications on a global scale.

Frequently Asked Questions(FAQ)

Q: What is X.400?A: X.400 is a suite of ITU-T (International Telecommunication Union – Telecommunication Standardization Sector) standards that defines global systems for electronic messaging services.Q: What does X.400 protocol do?A: X.400 protocol generally provides robust and comprehensive features for messaging over a network. It sets guidelines for the exchange of different types of messages including emails on networks.Q: What type of information does X.400 carry?A: X.400 carries a wide range of information, including email, attachments, and Non-Delivery Reports (NDR) between Message Transfer Agents (MTAs), and from MTAs to email clients.Q: Who uses X.400?A: X.400 is generally used by large organizations, governments, and militaries where there is a need for a high level of reliability, security, and control over the messaging system.Q: How is X.400 different from SMTP?A: X.400 and SMTP (Simple Mail Transfer Protocol) are both messaging protocols, but X.400 offers more complex features, such as Receipt Notifications, Content Integrity Checks, and Enhanced Security services. SMTP is simpler and more widely adopted on the internet.Q: Is X.400 outdated?A: While it’s true that X.400 is not as commonly used due to the rise of less complex protocols like SMTP, it is still relevant and used in various large organizations and specific industries where its robust features are required.Q: What is an MTA in X.400?A: MTA in X.400 stands for Message Transfer Agent. It is responsible for receiving and transferring messages in an X.400 system.Q: Can X.400 and SMTP work together?A: Yes, it’s possible for X.400 and SMTP to interoperate via gateways. These gateways can convert messages, allowing them to move between X.400 and SMTP systems.

Related Tech Terms

  • P1 Protocol
  • P7 Protocol
  • Address Resolution Protocol (ARP)
  • Message Handling System (MHS)
  • Message Store (MS)

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