Note that it would be wise to strip incoming messages of A-R headers
that claim to be from our own <admd-identifier>.
+There are two new variables: $arc_state and $arc_state_reason.
+
Receive log lines for an ARC pass will be tagged "ARC".
An option on the smtp transport, which constructs and prepends to the message
an ARC set of headers. The textually-first Authentication-Results: header
is used as a basis (you must have added one on entry to the ADMD).
+Expanded; if unset, empty or forced-failure then no signing is done.