Scenario
You notice that your email messages are being rejected when they attempt to route to Exclaimer, displaying a non-delivery report with a similar message:
Reason
You received this error message as your Exclaimer subscription is not using a per-tenant certificate. This configuration has been a default setup for the last 4 years and is now required by Microsoft when routing through a service similar to ours.
Resolution
To resolve this issue, you need to configure your Exclaimer subscription to use the per-tenant certificate. For more information, see how you can re-run the Microsoft 365 connector setup.