sendmail, 4th Edition

$-operators Are Tokens

As we progress into the details of rules, you will see that certain characters become operators when prefixed with a $ character. Operators cause sendmail to perform actions, such as looking for a match ($* is a wildcard operator) or replacing tokens with others by position ($1 is a replacement operator).

For tokenizing purposes, operators always divide one token from another, just as the characters in the master list did. For example:

xxx$*zzz    becomes  →   xxx  $*  zzz