We are aware of an issue that is preventing email from being sent to @hotmail.com and @live.com recipients. When sending an email to one of these domains, you may receive an email from “Mail Delivery System” with the subject “Mail delivery failed: returning message to sender” with the following message:
host mx4.hotmail.com [65.55.92.152]: 550 SC-001 (SNT0-MC2-F50) Unfortunately, messages from **.**.***.*** weren't sent. Please contact your Internet service provider since part of their network is on our block list. You can also refer your provider to http://mail.live.com/mail/troubleshooting.aspx#errors.
We are working hard to resolve this issue and we will keep you posted on any progress.
We apologize for any inconvenience this issue may cause.
This issue was fixed in the early evening on September 5th, 2013.