-This command returns exit status 0 if the message was successfully added
-to the mail directory, even if the message could not be indexed and
-added to the notmuch database. In the latter case, a warning will be
-printed to standard error but the message file will be left on disk.
+This command returns exit status 0 on successful mail delivery,
+non-zero otherwise. The default is to indicate failed mail delivery on
+any errors, including message file delivery to the filesystem, message
+indexing to Notmuch database, changing tags, and synchronizing tags to
+maildir flags. The ``--keep`` option may be used to settle for
+successful message file delivery.
+
+This command supports the following special exit status code for
+errors most likely to be temporary in nature, e.g. failure to get a
+database write lock.
+
+``75 (EX_TEMPFAIL)``
+ A temporary failure occured; the user is invited to retry.