Under open access every stranger is granted, so each forged or DMARC-failing
message logged a WARNING telling the operator to disable sender
authentication. Keep that warning (and hint) for the one verdict that points
at a server that never stamps the header; log every other unauthenticated
drop at debug, including an explicit failing verdict for an allowlisted
address. The reason string is lifted to _NO_AUTH_RESULTS_REASON so the
parser and the check share it.
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