The site won't let me send a invoice

If you encountered an error message specifically “Delivery Server Down”, this issue is related to email attachments on your sales orders or invoices, or it may depend on how your company's email address is configured.

Make sure your company email and all customer-facing email fields are complete, have the correct format, and don't have prefixes. Here’s how:

1. Go to Settings  and select Account and settings.
2. Select Company.
3. ( Correct email ) mycompany@mycompany.com.

Check out this article for further guidance: Fix the Delivery Server Down error in QuickBooks Online.

If you’re referring to something else, please don’t hesitate to reach out. We're always here to assist you.