The -d21.2 debugging switch tells
sendmail to show the current value of any
deferred-expansion macro (one that was declared with the
$& prefix). Each such macro that is
encountered in processing a rule prints as:
rewrite: LHS $&char => "value"
rewrite: RHS $&char => "value"
rewrite: LHS $&name => "value" V8.7 and above
rewrite: RHS $&name => "value" V8.7 and above
The char is the single-character name of the
macro, the name is either a multicharacter macro
name or a single-character name, and the value
is its current value. If that particular macro lacks a value, it will
print as (NULL). The LHS refers to the lefthand
side of the rule, and the RHS corresponds to the
righthand side. Deferred-expansion macros are described in Section 21.5.3.