Rules are like little if-then clauses, existing inside rule
sets, that test a pattern against an address and change the address
if the two match. The process of converting one form of an address
into another is called rewriting. Most rewriting
requires a sequence of many rules because an individual rule is
relatively limited in what it can do. This need for many rules,
combined with the sendmail
program's need for succinct expressions, can make
sequences of rules dauntingly cryptic.
In this chapter we dissect the components of individual rules. In the
next chapter we will show how groups of rules can be combined to
perform necessary tasks.