Please help. Sending an invoice gives me this error: Invoice undeliveredWe couldn’t reach some of your customers. Double check the email address and try again.

Thank you for letting us know about your experience when trying to send an invoice, Robert.

There's an ongoing issue when trying to send an invoice. Our customers are encountering the error message, Invoice undelivered. We couldn’t reach some of your customers. Double-check the email address and try again. Our Product Engineering team is aware of this and is now working on a fix. 

In the meantime, I suggest contacting our Customer Support Team, so you'll be added to the list of affected users and receive an email notification once we resolve the issue. 

You can reach our Customer Support for QBO by going to the Help icon at the top right of the account. Then, let them add you to Investigation No. 87823.

Follow the steps below: 

1. Go to the Help icon to connect to a live support agent. 
2. Provide us with some information about your concern.
3. Select the Contact Us button and provide some details about this concern, then on the Let's Talk button. 
4. You'll now be provided options on how to connect to our Customer Support.

You can check this article for more information about the different types of support we offer and their availability: How and when can I contact QuickBooks Online support?. 

In the meantime, you can download the invoice and send it manually using your email. Here's detailed guidance for creating and sending your invoices in QBO. Once you receive their payment, here's how to record invoice payments manually.

Let me know if you need further information about invoices in QBO. I'm always glad to help in any way I can. Have a great rest of the day!