Email Program Classifications - Red Hat ENTERPRISE LINUX 3 Reference Manual

Chapter 11. Email 161
For added security, it is possible to use SSL encryption for client authentication and data transfer
sessions. This can be enabled by using the service, or by using the
imaps /usr/sbin/stunnel
program. Refer to Section 11.5.1 Securing Communication for more information.
Other free, as well as commercial, IMAP clients and servers are available, many of which extend
the IMAP protocol and provide additional functionality. A comprehensive list can be found online at
http://www.imap.org/products/longlist.htm.
11.2. 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.
11.2.1. Mail Transfer Agent
A Mail Transfer Agent (MTA) transfers 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 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.
For more information on Sendmail, Postfix, and Fetchmail, refer to Section 11.3 Mail Transport
Agents.
11.2.2. 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 or Procmail.
mail
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.