Thanks for reaching out to us here in the Intuit Community, @newrivercomputing.
I appreciate you performing some troubleshooting steps to get this resolved.
One of the reasons you're getting this message is when you run QuickBooks as administrator. It's also possible there's another Outlook open in the background when sending invoices.
Make sure to close any open Outlook program, then QuickBooks is not running as administrator of the Outlook:
- Right-click QuickBooks Desktop icon, then select Properties.
- Go to the Compatibility tab.
- Unselect Run this program as Administrator.
- If Run this program as Administrator is grayed out, select the Show Settings for All Users.
- Click OK, then Exit.
- Make sure to restart the software.
If the same issue persists, you can try to perform other troubleshooting steps outlined in this article: Error QuickBooks is unable to send your email to Outlook.
I also suggest checking out the system requirements for needed for your version. This is to ensure the Outlook version is supported by the program.
Please keep me posted on how this goes. I'm always around whenever you need further assistance emailing invoices.