Hi there, @info661.
I can see that the undelivered invoices issue affects your business and your client's business. I've checked the investigation and it was already closed. And I'd like to share some troubleshooting steps to fix the issue.
If your clients still encounter the same issue, you'll want to check the status of the sent invoice and make sure the customer's email address is correct. For more guidance, feel free to check out this article: Fix undelivered invoices in QuickBooks Online.
You can also read this article for more troubleshooting steps if they're still getting the same issue: 3 solutions for you when customers aren't receiving your emails.
Your client may also want to view all their sales transactions in QBO. You can go to the Sales menu and select All sales.
Come back to this post and let me know how it goes, @info661. I want to make sure you're taken care of.
