Hi judy48,
Delivery errors can appear for a number of reasons, e.g. issues with the email server not recognising the email was received or QuickBooks not receiving a confirmation message from the email server etc.
If your customer is actually receiving the email that's fantastic but I can understand it would be frustrating that it says not delivered on your end.
If you experience this issue again can you please contact the Support Team so we can investigate further?
Thanks,
Rebecca
