27.4. Mail Transport Agent (MTA) Configuration
A Mail Transport Agent (MTA) is essential for sending email. A Mail User Agent (MUA) such as Evolution, Thunderbird, and Mutt, is used to read and compose email. When a user sends an email from an MUA, the message is handed off to the MTA, which sends the message through a series of MTAs until it reaches its destination.
Even if a user does not plan to send email from the system, some automated tasks or system programs might use the
/bin/mailcommand to send email containing log messages to the root user of the local system.
Red Hat Enterprise Linux 5 provides three MTAs: Sendmail, Postfix, and Exim. If all three are installed,
sendmailis the default MTA. The Mail Transport Agent Switcher allows for the selection of either
eximas the default MTA for the system.
system-switch-mailRPM package must be installed to use the text-based version of the Mail Transport Agent Switcher program. If you want to use the graphical version, the
system-switch-mail-gnomepackage must also be installed.
For more information on installing RPM packages, refer to Part II, “Package Management”.
To start the Mail Transport Agent Switcher, select System (the main menu on the panel) > Administration > Mail Transport Agent Switcher, or type the command
system-switch-mailat a shell prompt (for example, in an XTerm or GNOME terminal).
The program automatically detects if the X Window System is running. If it is running, the program starts in graphical mode as shown in Figure 27.1, “ Mail Transport Agent Switcher ”. If X is not detected, it starts in text-mode. To force Mail Transport Agent Switcher to run in text-mode, use the command
Figure 27.1. Mail Transport Agent Switcher
If you select OK to change the MTA, the selected mail daemon is enabled to start at boot time, and the unselected mail daemons are disabled so that they do not start at boot time. The selected mail daemon is started, and any other mail daemon is stopped; thus making the changes take place immediately.