Mail Delivery Agent
This is not
a true server (it is not using any port for accepting connections),
yet it is a vital part of the e-mail system, as it is implemented
on this server. The Mail Delivery Agent is responsible for delivering
messages received with various protocols to their destination. The
following diagram shows how the MDA operates:
Email messages sent
with various protocols are received by the your server. In the above
diagram you can see 3 communication types:
1) A user connects
to his/hers local mail server and send a message which is then relayed
through the Internet to your server with SMTP.
2) A user connects
directly to your server and send a messages (given that the user
has a local account), using the SMTP protocol
3) By using the
web interface, a user send a message by using the HTTP protocol.
When the server
receives a message, it will be placed in a database. As you can
see from the above diagram, there is no direct link between the
database and the user folders. This link is made through the Mail
The MDA will handle
each request from the database. If the destination is a local user
account, the e-mail will be delivered there. If the destination
is a remote account (on another server), the MDA will try to deliver
it. If the message can not be delivered, the Mail Delivery Agent
will place in the database a new message containing an explanation
for the delivery.
A message will fail
to reach its intended destination in the following situations:
the local user account was deleted before the message can be transferred
the remote account does not exist, or does not allow email to be
received from the specified source address
3) there is no route
to the destination server (the server may not exist, or its server
software is not properly configured).
If the message can
not be delivered to a remote address, the Mail Delivery Agent will
keep trying for approximately 10 hours. During this time, there
will be made 5 attempts with approximately 2 hours of delay.
After this time, if the message can not be delivered, there will
be a delivery failure notice, that will try to explain what's wrong
(it will include, if possible, a full transcript of the communication
between your server and the remote server).
In order to have a working e-mail system, you must run servers for
the following protocols: SMTP or ESMTP, POP3, MDA. For configuring
user accounts you must work with the web configuration (either by
running the Configuration server or by copying the web interface
in your main site).
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