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.