Thanks for alerting us of your concern, Office.

I understand the urgency of resolving this issue. As per checking, we are continuing to work on a fix regarding the Delivery Server Down error when sending invoices in QuickBooks Online (QBO). 

While the investigation is ongoing, you can opt to download the invoices as PDFs and send them manually to your customers through email. For the complete steps, please see this article: Download and save sales forms and statements in QuickBooks Online.

We’ll share further details as soon as we learn more. You may also check for updates on your end from time to time through this link: https://status.quickbooks.intuit.com/

We appreciate your patience while we're working on a resolution. If you have any further questions, please don't hesitate to reach out back to us.