X.400

X.400 is a suite of ITU-T recommendations that define the ITU-T Message Handling System (MHS).

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. 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.