OSI/MHS Orientation Guide

Building Your Message Handling System
OSI/MHS Orientation Guide—424829-001
1-4
X.400 Message Handling Systems
The protocols for communication among these components are labeled as follows in the
Recommendations and most other documents describing X.400:
•
P1 is the protocol between two MTAs.
•
P2 is the protocol between two UAs (for exchange of interpersonal messages across
the network).
•
P3 is the protocol between a UA and an MTA, when the UA requests services of the
MTA directly.
•
P7 is the protocol between a UA and a message store (MS).
If you currently use a non-X.400 messaging system, it almost certainly includes some
UA and MTA functions. To allow its applications to communicate across an X.400
network, you must develop or acquire an X.400 gateway, an application that allows
your system to emulate some features (such as routing functions) of an X.400 MTA and
that typically translates between X.400 and some other message format. One
specialized type of gateway, called an access unit, provides X.400 access to devices
such as telex or facsimile, to telematic services or physical delivery systems. An X.400
gateway can be collocated with the non-X.400 messaging system; alternatively, Compaq
provides several interfaces you can use to develop custom gateways on the NonStop
system.
You will hear more about these types of components when you read the task
descriptions later in this section. Figure 1-2 introduces the components in order to help
you define the tasks you will perform to build your messaging system.