report the specific properties used to the upstream notmuch project,
as a way of avoiding collisions in the property namespace.
+CONVENTIONS
+===========
+
+Any property with a key that starts with "index." will be removed (and
+possibly re-set) upon reindexing (see **notmuch-reindex(1)**).
+
+MESSAGE PROPERTIES
+==================
+
+The following properties are set by notmuch internally in the course
+of its normal activity.
+
+**index.decryption**
+
+ If a message contains encrypted content, and notmuch tries to
+ decrypt that content during indexing, it will add the property
+ ``index.decryption=success`` when the cleartext was successfully
+ indexed. If notmuch attempts to decrypt any part of a message
+ during indexing and that decryption attempt fails, it will add the
+ property ``index.decryption=failure`` to the message.
+
+ Note that it's possible for a single message to have both
+ ``index.decryption=success`` and ``index.decryption=failure``.
+ Consider an encrypted e-mail message that contains another
+ encrypted e-mail message as an attachment -- if the outer message
+ can be decrypted, but the attached part cannot, then both
+ properties will be set on the message as a whole.
+
+ If notmuch never tried to decrypt an encrypted message during
+ indexing (which is the default), then this property will not be
+ set on that message.
+
SEE ALSO
========