Apologies for the trouble. We're aware of this issue and have already contacted Microsoft to have the SMTP server IP delisted.
Details of the incident are available on our status page at: http://status.opalstack.com/
We'll update the status page when more information is available. If you would like to subscribe to automatic status updates via email you may do so at: https://status.opalstack.com/subscribe
As a possible temporary workaround, you can create a temporary mailbox on our US mail server, then configure your application and email clients to use that mailbox name for their outgoing mail authentication.