Welcome to the thread, Jennifer1102! It's wonderful to see you make your first post! I truly appreciate the information you've provided. Ensuring your invoices are successfully delivered is vital for effectively tracking your revenue data. I’m here to assist you with some troubleshooting steps to help resolve the undeliverable error message you’ve encountered, so you can manage your business smoothly and confidently.

Have you had a chance to verify your customer's email address? If you haven't already, I recommend following these steps to ensure it’s accurate:

1. Go to Customers & leads and select Customers.
2. Choose the customer you are experiencing issues with regarding invoice delivery.
3. Click Edit.
4. Check the email address and update it if needed.
5. Hit Save. 
6. Resend the invoice.

Right after, ask your customer to check their junk or spam folders if they didn't receive it in their inbox.

Next, you can clear your email address and re-enter it in your company settings to refresh the system. Here's the step-by-step guide to complete the process:

1. Go to Settings and pick Account and settings.
2. In the Company section, click Edit in Contact info.
3. Clear the Company email field and enter the desired address as is, even if it appears correct. (Note: Be sure not to use any extra characters or spaces before, within, or after the address you enter).
4. Hit Save, then Done. 
5. Send yourself a test transaction.
6. If you still don't receive the email, try using a different email address that has a different domain (for instance, if you were using a Gmail email address, try a Hotmail or Yahoo address instead).

For the comprehensive guide about the steps above, please see this article: Troubleshoot if customers aren't receiving emails from QuickBooks Online.

Moreover, would you like to enhance your customer interactions by personalizing your sales forms with specific details? If you're interested, I have some great resources to help you: Customize invoices, estimates, and sales receipts in QuickBooks Online.

I'm glad I was able to assist you today, Jennifer1102. You can always return here by commenting below if you have clarifications or questions about managing your invoices and sales transactions or related topics. I'll make sure to respond as soon as possible. Wishing you productive and successful business operations!