We haven't been able to deliver the message yet to the recipient because we can't connect to your organization's mail server. ...

We haven't been able to deliver the message yet to the recipient because we can't connect to your organization's mail server. We can't get its DNS record. 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 troubleshooting your DNS configuration.
Go to the connector page.
Choose the connector indicated in the previous section, and select Edit
Check the connector for typos in the smart host configuration or other wrong information.
If that doesn't fix the problem, you'll need to fix your DNS configuration. The smart host ({SmartHostName}) in your connector must have an A record. Alternatively, you can use IP addresses as the smart host.

 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.