Overview of X.400

X.400 is a suite of protocols developed by the International Telecommunication Union (ITU-T) and the International Organization for Standardization (ISO) for electronic messaging services. 

X.400 was designed in the 1980s to be a comprehensive standard for email exchange within large-scale organizations, government agencies, and the military, especially in regions where telecommunications infrastructure was not well-suited for internet-based email services like SMTP.

X.400 became particularly popular in Europe and some parts of Asia, as it offered features and capabilities beyond simple email delivery, with more stringent requirements for reliability, security, and non-repudiation.

Key Features of X.400:

1. Protocol Suite:

   • X.400 is not just one protocol but a suite of protocols for electronic messaging, covering message creation, submission, transmission, delivery, and storage.
   • Unlike SMTP, X.400 supports structured messages with more detailed control over the contents, sender/receiver information, and message handling requirements.

2. Network-Independent:

   • X.400 was designed to be network-independent, meaning it could work over various types of communication networks, such as X.25 (an older packet-switched network standard), public switched telephone networks (PSTN), or LAN environments.
   • It was particularly useful for countries where the Internet was not fully developed at the time, and organizations needed a reliable email system that could operate over existing telecom networks.

3. Hierarchical Addressing:

   • X.400 uses hierarchical addressing, which is more complex than the simple domain-based addressing used by SMTP. An X.400 address includes a detailed breakdown of the organization, country, department, and other elements to ensure precise message delivery.
   • Example of an X.400 address:

     C=US; A=ATTMAIL; P=COMPANY; O=SALES; S=SMITH; G=JOHN

     Where:
     • C: Country code
     • A: Administrative management domain (ADMD)
     • P: Private management domain (PRMD)
     • O: Organization
     • S: Surname
     • G: Given name

4. Message Types:

   • X.400 can handle multiple types of messages, including standard emails, voice messages, and binary files.
   • Messages can contain structured data, with strict rules for how information is formatted and conveyed, which can be essential for business and government communications.

5. Security and Reliability:

   • X.400 supports a range of security features, including authentication, encryption, and non-repudiation (guaranteeing that a sender or receiver cannot deny having sent or received a message).
   • It provides more robust tracking of message delivery and non-delivery reports than SMTP. Detailed delivery and read receipts can be generated to ensure that messages reach their intended recipients.

6. Store and Forward:

   • Like SMTP, X.400 operates using a store-and-forward model, meaning that messages are stored temporarily on intermediate servers while they are being routed to their final destination.
   • X.400 is highly reliable, with mechanisms for retries and detailed error reporting if a message cannot be delivered.

7. Multilingual Support:

   • X.400 was designed to support multiple languages and character sets, making it suitable for international communications.

Use in EDI (Electronic Data Interchange):

X.400 can also be used as a transport protocol for EDI messages, ensuring reliable and secure exchange of structured business data between trading partners. In this context, X.400's robust error handling, security features, and detailed delivery reports made it suitable for B2B communications, particularly in sectors like healthcare, finance, and logistics.

Summary:

X.400 was a comprehensive and powerful email protocol that provided security, reliability, and detailed message handling for organizations that needed more control over their communications than SMTP could offer. However, due to its complexity and the rise of Internet-based messaging, X.400 has largely been supplanted by SMTP, though it remains in use in certain specialized domains where security and detailed message reporting are critical.