I'm here to ensure your invoices get delivered promptly and get past the problems you've encountered in doing so, Damien.

The Undeliverable invoice error could occur if you have set up multiple addresses. If any of the ones fails when sending an invoice, regardless of the other one being successful, you'd still receive the said notice. We can handle these situations by initiating the procedures below.

To start, let's double-check if the Copy (Cc) and Blind Copy (Bcc) emails are accurate from the Account and settings menu. If applicable, you can remove them and turn off the Email me a copy at function instead, as they can trigger errors when sending invoices.

1. Go to the Gear icon, then Account and settings.
2. Select the Sales tab.
3. Tick the Pencil icon in the Messages field.
   
4. Modify the emails in the Copy (Cc) and Blind Copy (Bcc) sections if needed. You can also choose to retain one and clear the other email if you've entered multiple addresses for each section or remove them all and disable the Email me a copy at option.
5. Once done, hit Save.
   

After that, review the customer's profile and make sure the recorded email(s) are precise. If not, you can modify it, and if there are multiple, separate each with a comma (,). Refer to these steps to proceed:

1. Go to the Sales menu, then Customers.
2. Locate and select the customer(s) to whom you're trying to send the invoice from the list.
3. Click Edit and double-check the Email information.
4. Once done, hit Save.
   

On the other hand, the Unable to reach email address prompt will occur if you're using a trial account because you're only limited to 100 maximum emails using this version. However, if you're using QuickBooks Online Accountant (QBOA), you'll encounter this problem if you don't have a valid client. Given that, I recommend completing or availing a QuickBooks subscription to continue sending unlimited emails (invoices) or ensuring your clients are valid (QBOA). 

If that is not the case, let's initiate browser troubleshooting procedures to narrow down the result. Let's start by accessing your account and sending the invoice(s) using incognito mode:

Follow these keyboard shortcut keys that vary by browser type:

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

Once you encounter the same result, I recommend clearing your standard browser's cache and cookies. It helps eliminate excessive data build-up that affects the program's functionality. If the issue persists, you can consider using another supported browser.

Nevertheless, if you still get the same errors, I suggest contacting our Support Team to investigate this matter thoroughly. They're equipped to dig deeper into your concern and ensure accurate resolution.

I'm also attaching this article you might find handy when personalizing your sales forms in the software: Customize invoices, estimates, and sales receipts in QuickBooks Online.

You can always drop by this thread should you have other concerns about sending your invoices in QuickBooks. Our team is always delighted to assist.