Relevant Product: Exclaimer Cloud - Signatures for Office 365
You notice that your email messages are being rejected when they attempt to route to Exclaimer Cloud, displaying a non-delivery report with a similar message:
You received this error message as your Exclaimer Cloud 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.
To resolve this issue, you need to configure your Exclaimer Cloud subscription to use the per-tenant certificate - please see how you can do it by this link: How to re-run the Office 365 Connector Setup Wizard.