The last e-mail that came in for an order notification came in as spam.
I checked with Rackspace, and they say the messages are coming in clean to the server. They suspect that you are using an e-mail client like Outlook or Mac Mail. It may be that those clients are filtering them to spam. It may mean something has changed recently in the e-mail client's filtering. If this is the case, make sure that you have the notification e-mail address white-listed in you e-mail client.