Let's check to see why your invoices are not sent out, vaughancashmore.
If these are recurring invoices, please check if they have reached the End date,or if the Interval schedule was changed. Then, the Automatically send emails checkbox in the Options section should also be checked, so the invoices will be sent out.
If they are not recurring invoices, did you get an error message when you tried sending them? Was there a prompt or something?
You can do the basic browser troubleshooting steps to check if this is triggered by a browser issue.
You'll want to start by logging in to QuickBooks in a private window. This will disable your cache from saving new data, and it’s a good way to check browser issues. If you can send your invoices, you can go back to your regular browser and clear its cache. If it’s the same thing, please try other browsers to isolate the case.
Good job for doing the steps my colleague suggested, HughBrock. It's the basic troubleshooting for that kind of issue.
I also want to thank you for alerting us with the specific error message. Since the steps didn't work, I'd suggest reaching out to our customer care support so an agent can take a better look at it. They might open a new case to be submitted by our engineers for investigation. Here's how:
Click Help inside QuickBooks Online.
Select Contact us.
Enter "Delivery Server Down error when sending invoices" in the description field, then click Let's talk.