I have done my research and found that, when you have tried sending an email to the buyer, informing him about the delivery date of his order, you are receiving an error message stating 'Unreceived message: Recipient's email address is invalid '.
Please be informed that, whenever you get this error message, it means that, these messages are still available in their buyer account if the buyer comes to look, however it is the notification to their email inbox that they will not receive.