I just went to send a invoice to several different customers that are already on my quick books and the email is coming back as delivery failed

Your efforts to connect with the Community are much appreciated. I'd be happy to inform you of QuickBooks Online's email-sending issue.

We are aware that certain customers of QuickBooks Online are temporarily unable to send invoices due to the Delivery Server Down error. Our engineers are working diligently to fix this issue as soon as possible.

Check out for updates through this link: https://status.quickbooks.intuit.com/.

You can manually send invoices outside of QuickBooks in the meantime. Here's how:

1. Choose the Print or Preview icon after opening your transaction.
2. Tap on Download, then save the file.
3. Send the customer an email with the file attached.

To learn how to customize and add information to your sales forms moving forward, read this article: Customize invoices, estimates, and sales receipts in QuickBooks Online.

We value your patience and understanding. If you have any additional questions about QuickBooks, just click the Reply button or post in the Community again. Keep safe!