Update:
Our outbound IP addresses have been adjusted so that end-users are no longer impacted by the erroneously filed spam reports. You can resend any returned emails at this time and they should deliver as intended. If you experience any issues, please do not hesitate to reach out to our Support team.
Original Systems Notification Post:
Our engineering team has discovered that some email is being blocked due to erroneous spam reports filed with Microsoft. Customers on our Hosted Exchange service may be impacted when sending email to Hotmail, live.com, and outlook.com. Any emails that are not delivered will generate a non-delivery response. We are in communication with Microsoft to resolve this issue as quickly as possible. If you receive an NDR for an email sent, this email will need to be resent when the issue is resolved, it will NOT remain in the queue.