Its foundation is a configuration file that
defines the location and behavior of these other parts and contains rules for rewriting
addresses. A queue directory holds mail until it can be delivered. An aliases file
allows alternative names for users and the creation of mailing lists. Database files can
handle tasks ranging from spam rejection to virtual hosting.
1.4.1 The Configuration File
The configuration file contains all the information sendmail needs to do its job.
Within it you provide information, such as file locations, permissions, and modes of
operation.
Rewriting rules and rule sets also appear in the configuration file. They transform a
mail address into another form that might be required for delivery. They are perhaps
the single most confusing aspect of the configuration file. Because the configuration
file is designed to be fast for sendmail to read and parse, rules can look cryptic to
humans:
R $+ @ $+ $: $1 < @ $2 > focus on domain
R $+ < $+ @ $+ > $1 $2 < @ $3 > move gaze right
But what appears to be complex is really just succinct. The R at the beginning of each
line, for example, labels a rewrite rule. And the $+ expressions mean to matchone or
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1.5 Basic Parts of a Mail Message | 5
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