We understand the importance of sending invoices on time, which notifies your customers and allows them to pay as scheduled, Studio45.
 

Please know that this error commonly occurs when there's a problem with the customer's email provider or their email address itself. I suggest verifying their email or re-entering them on the Invoice page to narrow down the result. You can also ask for an alternative one to send the invoice to.
 

Aside from that, may I ask if you've already performed the suggestions provided in this thread? If so, and the issue persists, I recommend contacting our live support team directly. They can investigate this matter thoroughly in a safe environment using their designated tool.

Here's how:

1. Login to your QuickBooks account.
2. Go to the (?) Help icon, then Search tab.
3. Type the "Contact Us" keyword in the designated field. Then, press the Enter button on your keyboard.
4. Click the Contact Us option once it appears on your screen.
5. Choose you're preferred method of connecting with our live experts and follow the on-screen instructions.

It's also important to note that our live team has their specific availability. You can navigate to the Support Hours section of this link for more details: Get help with QuickBooks products and services.

Alternatively, you can download the invoice as a PDF file and send it to your customer via email:

1. Go to the Sales menu, then the Invoices tab.
2. Locate the invoice from the list and click its designated dropdown arrow under the Action column.
3. Select Print.
4. Click the Save icon beside the Print option.
   
   
5. Enter a File name and save it in your device's folder.
6. Attach the downloaded invoice to your email and send it to your customer.

Once your customers pay their invoices, you can then replenish or update their balances. You can use this article as a reference: Record invoice payments in QuickBooks Online.

If there's anything else that the Community team can help you with, don't hesitate to revisit this thread.