Tika does not properly detect certain emails as `message/rfc822` if they're slightly standards-non-compliant and begin with `Status: ` as the first header. I've added `Status: ` as a magic detection line in tika-mimetypes.xml.
This solves my problem and does not appear to cause unit test failures. I have not yet run the tika-batch tests.
As further information, the emails that are processed incorrectly come from dumps directly from various US public officials' mailservers. The dumps, I believe since they're not intended to be transmitted over the wire, sometimes are slightly non-compliant.
It's important to note that Tika (and the underlying library, James Mime4J) do properly parse these emails, despite the non-compliant header. The problem is getting Tika to detect the file as an email so that Mime4J gets chosen to parse it.
Pull request on Github at https://github.com/apache/tika/pull/40