On January 3 2022 at 20:47 UTC Microsoft began blocking mail sent from smtp.de.opalstack.com to domains that use the Microsoft 365 email system.
Microsoft's own IP reputation and junk mail reporting systems indicate that there are no problems with smtp.de.opalstack.com so we've reached out to them for answers.
We'll update this post as soon as more information is available.
In the interim as a possible workaround customers may create a new mailbox on our US mail server and use that temporarily for their outgoing email authentication.
It's been over a week and we've seen no blocks of the new server IP by MIcrosoft, so we consider this to be resolved.
We informed Microsoft of our findings and the corrective action we've taken but we've still received no further communication from them.
To work around this we've moved smtp.de.opalstack.com to a new IP address. Customers do not need to change any settings - the DNS change should propagate automatically within the hour.
After the DNS change has propagated, customers should be able to send mail to email addresses that are hosted by Microsoft.
We'll leave this incident open and will continue to monitor.
We've still not heard from Microsoft but we believe we've identified the activity that triggered the block and are taking action to mitigate it at this time.
We'll notify Microsoft of the actions we're taking and continue to hold on for a response.
We're still awaiting a response from Microsoft and have reached out to them as a reminder.
