Message Transfer Agent
The MTA is the message switch within an X.400 network. Numbers of MTAs connected together form the
message transfer system.
An MTA performs the following basic functions:
- It aaccepts messages submitted by message originators' U.A.s (User Agents) for transmission to the
appropriate recipients' UA or other MTAs or Aus.
- It accepts messages transmitted to it by other MTAs for transmission to recipients' UA or other MTAs
or Aus.
- It analyzes the recipient list in the message and performs routing decisions.
- If an address in a message points to a UA served by itself, it passes the message to the UA and
generates a delivery notification, if requested.
- If an address indicates that a message needs to be passed to another MTA, it relays the message to the
next MTA.
- If an address cannot be successfully resolved, it generates a nondelivery notifiction (NDN) for that
address.
The originator of a message causes their UA to pass a message to their local MTA. This MTA analyses the
address in the message and tranfers the message to the appropriate MTA. This MTA will again analyse the
address and will either pass the message to the appropriate UA or MTA. This continues, when an MTA
receives a message addressed to one of its user agents it passes it on to the UA and then generates a positive
delivery notification, which it addresses to the originator of the message. The MTAs cooperate in a similar
manner to transfer notification back through the network.
If a MTA is properly implemented, it can provide many more services. For example if a message is
multiaddressed, one or more MTAs can create copies of the message, each copy being delivered to a
different recipient.
Interfacing Two MTAs
We noted already interconnected MTAs form the MTS. Since different users and providers demand varying
levels of functionality and throughput from MTAs, they are not same. So the problem arises how to connect
all these different MTAs. The answer is as follows: When exchanging messages with other MTAs, they first
encode the messages and notifications in a particular format, the Back to X.400 functional model
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