This was in the FAQ on one of Comcast pages:
From Comcast: At this time, MSN and Hotmail are experiencing problems receiving email from some domains, including Comcast.net. If you are unable to send email to an msn.com or hotmail.com address, please try sending the message again at a later time. Microsoft is aware of this issue and is working to resolve the problem as soon as possible.
From Me: Uh I've tried sending the messages at later times and it still doesn't work.