If you are signing up with a hotmail account you need to add [email protected] to your safe senders list. I have no idea why this is being blocked. I moved the email to mandrill about 6 months ago and our reputation is excellent. If you use another email service such as Yahoo or Gmail it is suggested that you use that email instead of Hotmail. I am attaching a link to an excellent Wikihow article on how to add an email to your safe senders list. If you are still having issues with the registration process please submit a support ticket.