+For S-Expression output, lists are printed delimited by () instead of
+[]. Objects are printed as p-lists, i.e. lists where the keys and values
+are interleaved. Keys are printed as keywords (symbols preceded by a
+colon), e.g. (:id "123" :time 54321 :from "foobar"). Null is printed as
+nil, true as t and false as nil.
+
+This is version 4 of the structured output format.
+
+Version history
+---------------
+
+v1
+- First versioned schema release.
+- Added part.content-length and part.content-transfer-encoding fields.
+
+v2
+- Added the thread_summary.query field.
+
+v3
+- Replaced message.filename string with a list of filenames.
+- Added part.content-disposition field.
+
+v4
+- replace signature error integer bitmask with a set of flags for
+ individual errors.
+