Normally, sendmail macros are expanded (replaced
with their values) when the configuration file is read. For those
situations when a sendmail macro should not be
expanded, but rather should be used in rules as is, V8
sendmail offers the $&
prefix. For example, consider the following RHS of a rule:
Normally, when sendmail encounters this RHS in
the configuration file, it will recursively expand
$w into its final text value (where that text
value is your hostname, such as wash.dc.gov).
But because the M sendmail
macro is prefixed (here, with $&) it is not
expanded until the rule is processed.
The $& operator can be used in either the LHS
or RHS of a rule. The $& operator is described
in full in Section 21.5.3.