mx1.de.opalstack.com and mx2.de.opalstack.com are listed on the Hotmail block list. As a result, mail forwarded to Hotmail domains like hotmail.com and outlook.com is being bounced back to the sender.
We're working to have the servers removed from the block list and will update this post when that is done.
In the meantime, customers who have forwarding addresses which forward to Hotmail domains are advised to forward to an alternate address or direct mail to an Opalstack mailbox to ensure that they receive their mail. Instructions for updating email addresses are available in our user guide at: Editing Email Addresses