Welcome to the QuickBooks Community, iconicaccounting. I appreciate you for performing the steps and for reaching out to our QuickBooks Support Team to fix the issue about sending invoices. I can also see that things have been challenging for you with this problem and with this, I'll share an update about this matter and ensure you'll be routed to the right person to assist you further on this concern.

Please know that being unable to reach <user's email> when sending invoices has been recently tagged as an ongoing issue. Our product engineers are now working to fix this as soon as possible. 

If a ticket has been raised when you contacted our support, rest assured that you're now added to the list of affected users. You'll receive updates through email once the issue is resolved. You can also send or reply to them using the existing email to check for the status of this matter. Also, you may reach out to our support again if you need more details about the case.

Lastly, you may refer to this article to see details on how the Sales page gives you a great at-a-glance view of the status of your sales transactions, like open and paid invoices: View sales transactions.

Don't hesitate to click the Reply button below if you still have a concern related to invoices in QuickBooks. I'm always around to help, 

iconicaccounting. Have a great day!