X.400
（重定向自ADMD）
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, 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.