At this time Microsoft is blocking mail forwarded to hotmail.com and outlook.com addresses via our mx2.us.opalstack.com server. We believe we've identified the cause and are currently waiting on Microsoft to lift the block.
In the meantime, customers who forward mail from their Opalstack-hosted domains to a Hotmail or Outlook address may want to assign a Opalstack mailbox to their email address to assure that they do not miss any incoming messages.
We'll update this post when we've either received a reply from Microsoft or when we can confirm that the block has been lifted.