We haven't been able to deliver the message to the recipient yet due to a problem on your organization's email server. Validation ...

Office 365

We haven't been able to deliver the message to the recipient yet due to a problem on your organization's email server. Validation for your server's certificate failed. Please review these details:

Recipient: {Recipient}
Email server name: {DestinationMailServer}
Connector name: {ConnectorName}
Technical error message: {MessageDetail}

To fix the problem, we recommend that you verify your connector settings before checking for a certificate failure.Go to the connector page.
Choose the connector indicated in the previous section, and select Edit
Check the connector configuration for typos or wrong information.
If that doesn't fix the problem, you'll need to fix the issue on your organization's email server based on the error in the previous section.

We'll keep trying to send the message for a total of 48 hours. After that, Office 365 will stop trying to send the message and will return a non-delivery report (NDR) to the sender.

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