It is possible to have rewrite rules whose application is dependent upon whether or not the address being rewritten was the result of an alias. (Note that inherently, only envelope To addresses can possibly be the result of an alias.) This can be useful in the case, for instance, where certain rewrite rule(s) should be applied only before alias expansion has occurred, while other rewrite rule(s) should be applied only after alias expansion has occured.
Alias-sensitive rewrite checking is triggered by the presence of the
$; control sequence in the template part of a rule. The
$: control sequence means that the rewrite rule will match only if the address being rewritten is the result of an alias. (So note that such a rewrite rule also is implicitly a
$E$F rewrite rule---it can only ever match envelope To addresses.) The
$; control sequence means that the rewrite rule will match only if the address being rewritten is not the result of an alias.