The
accept_unresolvable_domains feature (FEATURE(accept_unresolvable_domains)) allows all mail to be received, even when the
domain part of the envelope sender address cannot be looked up. This
feature is implemented in rules, in part, by using the
$={ResOk} class macro to hold a value that
indicates that an unresolved, envelope-sender address is acceptable.
The $={ResOk} class macro is strictly intended for
use by this feature and should not be used for anything else, nor be
modified in any way.
