We haven't been able to deliver the message to the recipient yet. Office 365 received an error because validation failed ...

We haven't been able to deliver the message to the recipient yet. Office 365 received an error because validation failed for the remote server's certificate. Please review these details:

Recipient: {Recipient}
Email server name: {DestinationMailServer}
Email Server IP address: {DestinationIP}
Technical error message: {MessageDetail}

The recipient won't be able to receive messages until their admin fixes the issue with the server. In the meantime, we recommend contacting the recipient another way.

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.