The $@ operator, when used to prefix the RHS,
tells sendmail that the current rule is the last
one that should be used in the current rule set. If the LHS of the
current rule matches, any rules that follow (in the current rule set)
This $@ prefix also prevents the current rule from
calling itself recursively. To illustrate, consider the following
R $* . $* $@ $1
The idea here is to strip the domain part of a hostname, and to
return just the host part. That is, if the workspace contains
wash.dc.gov, this rule will return
wash. The $@ prefix to the
RHS tells sendmail to return the rewritten
workspace without processing any additional rules in the current rule
set, and to allow the LHS to match only once.
Note that the $@ prefix can prefix only the RHS.
This operator is described further in Section 18.7.3 of