I am receiving an invoice delivery issue notice but some clients are definitely receiving their invoices because they pay them. How do I know who is getting them and not?

Thank you for visiting the QuickBooks Community, greatpanes11. Please know that the option to see who is getting the sent invoices and who is not is currently unavailable. However, let me provide alternative steps to achieve your goal.

QuickBooks has the feature to check the status of the transaction you've emailed or delivered to a customer. I suggest visiting the Invoice activity section to confirm if the entry appears as Viewed. This indicates that the invoices were successfully received by the customer.

Here's how:

1. Click the Sales menu on the left panel.
2. Go to the Invoices tab.
3. Locate each transaction and click it so the Invoice activity page appears. 
4. Scroll down to check if Viewed is already on the green status.

On the other hand, you have to reach out to your customers to confirm from them if they received their invoices. You can then verify to whom you need to resubmit the transaction.

Lastly, refer to this article if you need steps on how to fix the Delivery Server error that you might encounter in QBO: Fix the Delivery Server Down error in QuickBooks Online.

If you need any more assistance with sending invoices in QBO, don't hesitate to add a comment below. I'm always here to help. Stay safe and enjoy the rest of your day, greatpanes11.