Our domain is being blocked by the outgoing Outlook.com/Hotmail.com servers. When someone using one of these accounts tries to send us an email, they get a a bounce back message.
Subject: Delivery Status Notification (Failure)
Body: This is an automatically generated Delivery Status Notification. Delivery to the following recipients failed.
We have a spam filter that we can look at to see why an email was rejected. However, the emails are not even hitting our server. They are being blocked on the Outlook.com side.