MS email services (Hotmail, Live and Outlook) all appear to be blocking RHP mail.
...for policy reasons. Reasons for rejection may be related to content with spam-like characteristics or IP/domain reputation.
I will be investigating over the weekend how this can be reversed, but it almost certainly will take time. If the email reminders are essential, I would recommend trying an alternative such as Gmail for the meantime. (Not a real solution, I know)