X.400 message handling system: the remote user agent

The X.400 Recommendations of the CCITT establish specific criteria for packaging and addressing global electronic mail. X.400 is a communications application that resides at layer 7 of the open system interconnect (OSI) seven-layer reference model. Each layer performs a specific function. The primary objective of the article is to identify technical issues associated with allowing remote users access to the electronic mail system, including how dissimilar architectures pass information, the need for reliable communication protocols that reside below the X.400 application, how a system operator can submit and receive messages, and the need for a standard message structure. The issues identified provide the reader with useful information necessary for implementing the remote user agent (RUA) into an X.400 network. A secondary objective is to identify how remote users compose, edit, and store messages, as well as gain access to other systems for sending and receiving messages via LANs and WANs. A tertiary objective of is to identify where remote users fit into a global electronic mail system. The article provides a detailed understanding of how remote users can access the X.400-based electronic mail system for sending and receiving mail.