Rule sets in the configuration file, like subroutines in a program,
control the sequence of steps sendmail uses to
rewrite addresses. Inside each rule set is a series of zero or more
individual rules. Rules are used to select the appropriate delivery
agent for any particular address, to detect and reject addressing
errors, to transform addresses to meet particular needs, to validate
addresses and headers for the purpose of rejecting spam, and to make
In this chapter we will cover all aspects of rule sets, showing that
rule sets are called in particular orders and explaining why this is
We will explain many of the rules that typically appear in rule sets.
But be forewarned: the examples of rules in this chapter are only
explanatory. Your sendmail.cf file is likely to
have rules that are somewhat different. Copying or using these
examples, without first understanding the underlying principles, can
cause email to begin to fail.