If my customers do not open a email with invoice the next one I send will back up with the previous one rather than sending a new email. Invoices are being missed

Hi rainbowpristine!

I tried sending an invoice and not opening it in the email, and then sent another one. They're showing separately in the mail and in the customer's transaction list. Can you please elaborate more about this "backing-up" situation? Do you mean when you open and send the second invoice, QuickBooks will send the previous one instead? Thanks in advance.