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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.[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.
- ^ Ned Freed (13 July 2021). "Ticket #14". Emailcore (Mailing list). Retrieved 15 July 2021.
X.400 has changed from "the mail system we'll all be using someday" to "only used in limited government/military cases, and even those are disappearing"