Options define the sendmail environment. All of the option values are used directly by the sendmail program. There are nearly 100 options, but a few samples from the Red Hat sendmail.cf file can illustrate what options do.
Listing 5.3: Sample sendmail.cf Options
# location of alias file O AliasFile=/etc/aliases
# Forward file search path
# timeouts (many of these) O Timeout.queuereturn=5d
These options all have something to do with sendmail functions that have already been discussed. The first option command (O) sets the location of the aliases file to /etc/aliases. The second option defines the location of the .forward file. Notice the $z and $w included in this option. These are macro values. The $w macro contains the computer's hostname, indicating that it is possible to use the computer's hostname as a filename extension on a .forward file. Given the fact that you already know that the .forward file is found in the user's home directory, you can guess that the value of the $z macro is the user's home directory.
The last two options in the example relate to processing the queue of undelivered mail. The first of these options tells sendmail that if a piece of mail stays in the queue for five days (5d), it should be returned to the sender as undeliverable. The second of these options tells sendmail to send the user a warning message if a piece of mail has been undeliverable for four hours (4h). Many of the options in the sendmail.cf file set the timer values used by sendmail.
File locations and other things that vary based on the operating system being used are handled in the options section. The m4 macros build a sendmail.cf customized for the target operating system. Because of this, the options in the sendmail.cf file that comes with your Linux system are probably correct for that system.
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