We haven't been able to deliver the message to the recipient yet because we can't connect to the recipient email server. ...

We haven't been able to deliver the message to the recipient yet because we can't connect to the recipient email server. Either the remote server is not running or the remote email server's firewall is blocking Office 365 from sending messages. Please review these details:

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

To fix the problem, the firewall administrator needs to open port 25.
For more information, see Network ports for clients and mail flow in Exchange 2013. The recipient's email server needs to allow messages from the IP addresses listed in Exchange Online Protection IP addresses.

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.

Unfortunately, Office 365 support won't be able to resolve this issue because it's due to a problem that's outside the Office 365 service.