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Chapter 11. Email
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
program. Refer to
Section 5.1, "Securing Communication"
for more
/usr/sbin/stunnel
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.
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.
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 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.
For more information on Sendmail, Postfix, and Fetchmail, refer to
Section 3, "Mail Transport
Agents".
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
mail
Procmail.
188
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