Have you seen this article: http://blogs.technet.com/b/isablog/archive/2010/08/24/unable-to-receive-e-mails-from-the-internet-using-e-mail-protection-feature-on-forefront-tmg-2010.aspx
I am about to troubleshoot two different deployments with this issue today, I get back to you when I have checked this out.
