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.