One of our DE MX servers, mx2.de.opalstack.com on IP 178.162.221.165, was recently blocked by Microsoft. As a result, mail sent to addresses which forward to Microsoft services like hotmail.com, outlook.com, and their international variants would be bounced back to the original sender with an error message.
We've already resolved this issue with Microsoft but they say that it may take 24-48 hours for the block to be fully lifted.
We'll update this post after we've seen no further blocks in a 24 hour period.