Sending invoices to the customers is a critical task that can't be avoided. However, it can be exasperating when you can't send invoices in QuickBooks for certain reasons. At this moment, a pop-up appears on the screen saying "QuickBooks is unable to send your emails." The reasons why you are failing to send invoices could be a misconfigured firewall or antivirus settings. wrong internet connection settings, improper setup of email settings...