X.400 is a suite of ITU-T Recommendations that define standards for Data Communication Networks for Message Handling Systems (MHS) — more commonly known as "email".
At one time X.400 was expected to be the predominant form of email, but this role has been taken by the SMTP-based Internet e-mail. 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.
Read more about X.400: History, Addressing