Developed in cooperation with the ISO, the X.400-series recommendations specify OSI standard protocols for exchanging and addressing electronic messages.The companion F.400-series of recommendations define Message Handling Services built on Message Handling Systems (MHS), as well as access to and from the MHS for public services.For example, messages (email) exchanged among people is referred to as Interpersonal Messaging (IPM); electronically structured business documents (e.g., invoices, purchase orders, dispatch advice, etc.) exchanged among trading partners’ computers fall under the EDI protocols.As with most ISO standards dealing with application-level networking, X.400 failed to compete successfully with SMTP, the Internet-based equivalent in North America.The ITU-T Recommendations define specific protocols for a wide range of communication tasks.
All ITU-T recommendations provide specific terms for descriptions of system entities and procedures.
As X.400 inter-domain relay services were assumed by ITU to be run by PTTs, X.400 incorporated fields for the automated transfer of messages between X.400 and other PTT services, such as Telex, facsimile and physical postal mail. X.400 has been extended for use in military applications (see MMHS) and aviation (see AMHS).
An X.400 address is technically referred to as an Originator/Recipient (OR) address.
There are no longer any X.400 default proxy e-mail addresses in Exchange Server 2007.
Important features of X.400 include structured addressing, ASN.1 binary code enabling multimedia content (predating and more efficient than MIME), and integrated security capabilities. X.404 | ISO/IEC 10021-11), but the initial misconception that X.400 required PTT relay services, coupled with PTT volume-based charges for these, were factors that inhibited the widespread uptake of X.400.