On December 18 2021 at 19:30 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.
We've still not received confirmation from Microsoft that the block was lifted, but we've seen no further blocking activity since 23:50 UTC on 20 December 2021 so we consider this incident to be closed.
Microsoft began accepting mail from smtp.de.opalstack.com as of 23:50 UTC on 20 December 2021.
We're still awaiting official confirmation that the block has been lifted.
