Hi Intrinsicreef,

Let's check to see why your customer's invoice wasn't sent.

You can try sending an email to your client outside QuickBooks and see if it bounces back. If it will, you can inform them about it. If it won't you can update your QuickBooks to fix any minor issues that may have caused this behavior. Then, resend the invoice. You can also try sending the invoice to yourself.

If the invoice will still show as undeliverable, please contact our QB Desktop Support to check this further using more tools.

You can always go back to this thread if you have other questions in mind. Take care always!