Email Program Classifications; Mail Transport Agent; Mail Delivery Agent; Mail User Agent - Red Hat ENTERPRISE LINUX 5 - DEPLOYMENT Deployment Manual

Chapter 24. Email
More details on dovecot can be found online at http://www.dovecot.org.

24.2. Email Program Classifications

In general, all email applications fall into at least one of three classifications. Each classification plays
a specific role in the process of moving and managing email messages. While most users are only
aware of the specific email program they use to receive and send messages, each one is important for
ensuring that email arrives at the correct destination.

24.2.1. Mail Transport Agent

A Mail Transport Agent (MTA) transports email messages between hosts using SMTP. A message
may involve several MTAs as it moves to its intended destination.
While the delivery of messages between machines may seem rather straightforward, the entire
process of deciding if a particular MTA can or should accept a message for delivery is quite
complicated. In addition, due to problems from spam, use of a particular MTA is usually restricted by
the MTA's configuration or the access configuration for the network on which the MTA resides.
Many modern email client programs can act as an MTA when sending email. However, this action
should not be confused with the role of a true MTA. The sole reason email client programs are capable
of sending email like an MTA is because the host running the application does not have its own MTA.
This is particularly true for email client programs on non-UNIX-based operating systems. However,
these client programs only send outbound messages to an MTA they are authorized to use and do not
directly deliver the message to the intended recipient's email server.
Since Red Hat Enterprise Linux installs two MTAs, Sendmail and Postfix, email client programs are
often not required to act as an MTA. Red Hat Enterprise Linux also includes a special purpose MTA
called Fetchmail.
Section 24.3, "Mail Transport
For more information on Sendmail, Postfix, and Fetchmail, refer to
Agents".

24.2.2. Mail Delivery Agent

A Mail Delivery Agent (MDA) is invoked by the MTA to file incoming email in the proper user's mailbox.
In many cases, the MDA is actually a Local Delivery Agent (LDA), such as mail or Procmail.
Any program that actually handles a message for delivery to the point where it can be read by an
email client application can be considered an MDA. For this reason, some MTAs (such as Sendmail
and Postfix) can fill the role of an MDA when they append new email messages to a local user's mail
spool file. In general, MDAs do not transport messages between systems nor do they provide a user
interface; MDAs distribute and sort messages on the local machine for an email client application to
access.

24.2.3. Mail User Agent

A Mail User Agent (MUA) is synonymous with an email client application. An MUA is a program that,
at the very least, allows a user to read and compose email messages. Many MUAs are capable of
retrieving messages via the POP or IMAP protocols, setting up mailboxes to store messages, and
sending outbound messages to an MTA.
MUAs may be graphical, such as Evolution, or have a very simple, text-based interface, such as mutt.
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