We haven't been able to deliver the message to the recipient yet. We can't deliver the message because Office 365 can't get ...

We haven't been able to deliver the message to the recipient yet. We can't deliver the message because Office 365 can't get the MX record information for the recipient domain. Please review these details:

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

To fix the problem, the recipient's admin needs to update the MX record for their domain. They can do that at their domain's host website.
Unfortunately, Office 365 support won't be able to fix this problem because the recipient's configuration isn't working, not yours.

For more information about MX records and DNS, see DNS basics.