It's not a toast message. It's very similar in that it pops up like a toast message, but it's for a new incoming email. I have toast messages disabled in the Look and Feel options, and tried turning it on/off again and it made no difference. I can disable the pop up if I disable all notifications, so it's somehow tied to it. There's just not a way to disable the popup and keep the the status bar icon as far as I can tell.