At one time, the designers of X.400 were expecting it to be the predominant form of email, but this role has been taken by the SMTP-based Internet e-mail.
[1] Despite this, it has been widely used within organizations and was a core part of Microsoft Exchange Server until 2006; variants continue to be important in military and aviation contexts.
Developed in cooperation with ISO/IEC, the X.400-series recommendations specify OSI standard protocols for exchanging and addressing electronic messages.
These are modelled as conceptual protocols that treat P1 and P3/P7 as providing an underlying reliable transport of message contents.
[3] Important features of X.400 include structured addressing, ASN.1 binary code enabling multimedia content (predating and more efficient than MIME), and integrated security capabilities.
From the late 1980s, many major countries committed to the OSI stack, via GOSIP - Government Open Systems Interconnection Profiles.
In North America, many large defense contractors and universities had also already committed to the Internet and TCP/IP standards, including SMTP for email.
There, X.400 is still used in some applications, such as the military, intelligence services and aviation, mainly because the X.400 protocol supports encryption, and its functions for integrity and security were developed and deployed much earlier than their SMTP counterparts (S/MIME, PGP and SMTP-TLS).
In Europe, South America, and Asia, X.400 is quite widely implemented[citation needed], especially for EDI services.
This was in contrast to the "real" postal service, where even partial addresses would be routed to a dead letter office where they will attempt to deliver it even if some of the information is missing or wrong.
This meant that as long as the message reached the service provider, indicated by the "Administration Management Domain" (ADMD) portion of the address, the system would likely know of the user in question.
This is the dominant model today, where companies use an internal server, or even more commonly, use a provider like Microsoft 365, or Gmail, which is invisible outside the organization, and even to the users.
This multi-part addressing system also led to the format being complex; users were not sure which fields were important and tended to provide all that they could.
This made trivial things, like printing the address on a business card or typing it into the email client more difficult than simpler systems like those found in SMTP.
For instance, each Administration Management Domain (service provider) could optionally upload their directory to a shared X.500 server, and then allow this database to be searched by the X.400 User Agents during email creation, thereby avoiding needing to know anything about the address other than the recipient's name and some sort of organizational name like a company.
[11] Additionally, the goal of providing a universal address database was fundamentally flawed by the time it was proposed.
However, this was simply not the case in the 1980s; at that time, email was often associated with business or government users, and those organizations treated their member addresses as valuable, or even confidential.