A standard for email developed by the International Standards Organization. The development of this standard was a long process and it was released just as the Internet was exploding in size and well after a number of TCP-IP-based mail programs had been released. The consequence is that the number of X400 users is very much smaller than users of protocols such as SMTP, and is confined to governmental users. X400 is associated with a sister standard, X500, which specifies directory services. It is expected that this standard will be obsolete by 2005.