OSI/MHS Orientation Guide

Building Your Message Handling System
OSI/MHS Orientation Guide—424829-001
1-3
X.400 Message Handling Systems
As defined by X.400, a message handling system has two major components, illustrated
in Figure 1-2:
•
A user agent is a processing entity that acts on behalf of a person or an application
to submit and receive messages. It uses the services of a message transfer agent,
either directly or through a message store. In addition to providing facilities for
submitting and retrieving messages, user agents often provide support for
composing, decomposing, adding attachments, scanning, sorting, forwarding, and
filing messages locally, but those features are outside the scope of the X.400
Recommendations and ISO standards.
•
A message transfer agent (MTA) is a processing entity that receives messages
from user agents (or from other MTAs) and routes the messages across the network
to their destinations. At its destination, a message is either delivered directly to a
user agent or deposited in a message store for later retrieval.
In Compaq OSI/MHS, multiple processes running in parallel act as one logical MTA;
you can write your own user agents (UAs) or acquire them from other sources.
Figure 1-1. Linking Mail Systems Across a Heterogeneous Network
001CDT .CDD
Transfer
and PS Mail
Company X
EDI
Telematic
Services
Company Z
EDI
Company Y
Mail
UNIX Mail
LAN E-mail System N
X.400
Network
SprintMail
X.400
Legend
= X.400 Gateway