This chapter provides an overview of the MHS (Message Handling System).  The
main aspects of the MHS requiring introduction are: the function model,
the message structure, and delivery reporting.  

\section{Functional Model \label{x400-fm}}

\begin{figure} \centering
 \input{x400_model}
  \caption{MHS Functional Model}
  \label{fig:x400-model}
\end{figure}

A functional model of the MHS is shown in figure \ref{fig:x400-model}.
The MHS is a collection of MTAs (Message Transfer Agents), 
MSs (Message Stores), UAs (User Agents), and AUs (Access Units).  MTAs perform 
the store-and-forward message transfer function.  MSs provide storage for 
messages.  UAs enable users to access the MHS, and AUs provide
links to other communication systems (e.g., the postal system).  A more
detailed description of each of these entities follows.
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MTAs comprise the MTS (Message Transfer System), the principal 
component of the MHS.  A 
message is submitted to an MTA by an originating UA, MS or AU, transferred to 
the recipient MTA(s), and delivered to one or more recipient UAs, MSs, or AUs.
If the message is addressed to multiple recipients, the appropriate MTAs
perform any splitting (i.e., replicating) of the message needed for
delivery to each recipient.
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Messages are transferred between MTAs on a cooperating store-and-forward
basis.  Since no end-to-end association is required, the MTA 
serving the message recipient need not be active when 
the message leaves the originating MTA.  The message may be stored at a
relay (i.e., intermediate) MTA until the recipient MTA becomes 
operational.  
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MTAs transfer messages whose content may be encoded in any format.
MTAs neither examine nor modify the content of messages except when
performing conversion.  Conversion increases the effectiveness of the MHS 
by allowing users to submit messages in one encoded format (e.g., telex), 
and have them delivered in another encoded format (e.g., IA5).  A UA can
register with the MTA the encoded information types that may be delivered, 
and request the MTA to perform any required conversions.
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The UA is the MHS component that enables a user to access the MHS,
for both the origination and reception of messages.
When submitting messages, the UA supplies to an MTA, either directly or
indirectly via an MS, the message content, the address(es) of the message 
recipient(s), and the MTS services being requested.  The message content is
the information that the originator wants transferred to the message
recipient(s).  The address and service request data are used by
the MTS to deliver the message.  When receiving messages, the UA may accept
delivery of messages directly from an MTA, or it may employ an MS
to accept delivery of messages, and retrieve them from the MS
at a later time.
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The MS is an optional MHS component that acts as an intermediary between a 
UA and MTA.  The MS often co-resides with the MTA serving it.  The primary 
purpose of the MS is to provide a repository for the delivery of messages.  
The UA can retrieve messages from this repository.  By using an MS to accept 
delivery of messages, a UA is not required to be constantly available.  This 
is especially useful for UA applications implemented on personal computers, 
which are typically turned off at night.  The MS may also submit and forward 
messages on behalf of the UA, and notify the UA at the time of message 
delivery.
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The AU is the MHS component that provides a gateway between the MHS and 
another communication system.  AUs may, for example, provide 
intercommunication with telex, teletex, and facsimile systems.  Another AU, 
the PDAU (Physical Delivery Access Unit) enables MHS users to send messages 
to users residing on a physical delivery system, such as the Postal Service.  

\section{Mesage Structure \label{x400-ms}}

The structure of an MHS message is shown in figure \ref{fig:x400-message}.  
It consists of a message envelope and a message content.  As with a 
postal message, the envelope represents the information required by
the MTS to deliver the message, such as the address(es) of the recipient(s)
and any special handling instructions.  Many X.400 security parameters are
transferred on the message envelope.  The message content represents the 
information that the originator wants conveyed to the message recipient(s).

\begin{figure} \centering
 \input{x400_message}
  \caption{MHS Message Structure}
  \label{fig:x400-message}
\end{figure}

\section{Delivery Reporting \label{x400-dr}}

The basic X.400 messaging service provides notification of 
message non-delivery.  When a message cannot be delivered to a recipient, a
non-delivery report is generated and returned to the originator.
The content of the non-delivery report contains status
information about the subject message.  
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The MT service also provides notification of delivery as an optional 
service.  If a message originator requests acknowledgement of successful 
delivery, a delivery report is returned to the originator by the 
recipient's MTA upon delivery of the subject message.
