Just to confirm, did you get any error message when emailing invoices? If so, may I know what specific error you received? In the meantime, let's perform some troubleshooting steps that can help us isolate this.
To start off, make sure that QBDT is not running as administrator. Here's how:
1. Close your QBDT, then right-click the QuickBooks icon, then select Properties.
2. Choose Compatibility Tab and deselect Run this program as Administrator.
3. Click Okay, then Exit and reopen QuickBooks.
Next is to check your email set up in the Preferences. Here's how:
1. Go to Edit menu at the top, then pick Preferences on the drop-down.
2. Choose Send Forms on the left panel, then go to My Preferences tab.
3. Set Send e-mailusing to the option that you want to use and select OK.
4. Close all other programs, then restart your computer.