Why does delivery error undeliverable mean when trying to sent invoices

Hi there, @renee11_2. Thanks for posting in the Community. I'd be glad to help you out.

This error appears if the email address is incorrect or there's an issue with the recipient's email server. It could also be a browser issue.

There are times that the system occasionally behaves strangely when the browser frequently accesses pages. It could be due to corrupted cache files in your browser. 

Let's perform some troubleshooting steps to isolate the issue. First, use a different browser or a private window to help you fix this so QuickBooks Online runs smoothly and effectively. 

You can use these shortcut keys to access a private window:
 

• Mozilla Firefox or Microsoft Edge: CTRL + Shift + P
• Google Chrome: CTRL + Shift + N
• Safari: Command + Shift + N
   

Once done, try sending your invoices again. If it works without error, go back to your regular browser and clear your browser's cache. This fixes unexpected behavior in the program and to have a clean slate.

Otherwise, you can try other supported web engines to learn what computer and browser are needed for the best experience when using QuickBooks.

If the issue persists, make sure to check your company email is correct and that all customer-facing email fields are complete with the right format. Ensure that it doesn't contain prefixes.

You can also turn on or off the Attach Invoice as PDF by following these steps:
 

1. Select the Gear icon.
2. Under Your Company, select Account and Settings (or Company Settings) and then select Sales.
3. In the Online delivery section, select or uncheck PDF Attached.
4. Select Save then Done at the bottom.
5. Send your forms.

Also, you need to ensure there are no special characters on your invoice and check the file size of your attachment if there are any. 
 

You can check out this article for the common instances and steps to resolve this error: Delivery Errors when trying to email a sales form. 

In case you want to personalise and add specific info to your sales forms, feel free to visit this article: Customise invoices, quotes, and sales receipts in QuickBooks Online.

I'm still here if you need further assistance with sending an invoice in QuickBooks Online. Stay safe and have a good one!