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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.
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 Mozilla Mail, or have a very simple, text-based interface, such
as
.
mutt
3. Mail Transport Agents
Red Hat Enterprise Linux includes two primary MTAs, Sendmail and Postfix. Sendmail is
configured as the default MTA, although it is easy to switch the default MTA to Postfix.
Tip
For information about how to switch the default MTA from Sendmail to Postfix,
refer to the chapter called Mail Transport Agent (MTA) Configuration in the Red
Hat Enterprise Linux System Administration Guide.
3.1. Sendmail
Sendmail's core purpose, like other MTAs, is to safely transfer email among hosts, usually using
the SMTP protocol. However, Sendmail is highly configurable, allowing control over almost
every aspect of how email is handled, including the protocol used. Many system administrators
elect to use Sendmail as their MTA due to its power and scalability.
3.1.1. Purpose and Limitations
It is important to be aware of what Sendmail is and what it can do, as opposed to what it is not.
In these days of monolithic applications that fulfill multiple roles, Sendmail may seem like the
only application needed to run an email server within an organization. Technically, this is true,
as Sendmail can spool mail to each users' directory and deliver outbound mail for users.
However, most users actually require much more than simple email delivery. Users usually want
to interact with their email using an MUA, that uses POP or IMAP, to download their messages
to their local machine. Or, they may prefer a Web interface to gain access to their mailbox.
These other applications can work in conjunction with Sendmail, but they actually exist for
Mail User Agent
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