An empty recipient likely means that we couldn't extract a recipient email from the bounce message or, even if we could find one, we couldn't verify that it was a "trusted" value (which means we couldn't extract the hash to indicate this).
The type of unknown means it wasn't a known type of email (bounce, auto reply, etc), so it could be a "normal" email or even something like spam. You can view the raw email contents with the button the right and that may give more details.