I understand the importance of sending an invoice to your customers through email, @SupplyBright. Rest assured, I'm here to assist you in fixing this issue so you can send invoice emails to your customers again.

First, let's use QuickBooks Tool Hub to troubleshoot your QBDT app. Make sure to close QuickBooks before doing this. Please check this article for more details: Fix PDF and Print problems with QuickBooks Desktop.

Here's how:

1. Download the most recent version (1.6.0.3) of the QuickBooks Tool Hub. If you have the app already, proceed to the next step.
2. Open the QuickBooksToolHub.exe file.
3. Follow the installation steps and agree to the terms and conditions.
4. Run the app after the installation.
5. In the QuickBooks Tool Hub, select Program Problems.
6. Select QuickBooks PDF & Print Repair Tool. Please wait for it to finish.
7. Print, email, or save as a PDF from QuickBooks Desktop again.

If the problem persists, you can reset your temporary folder permissions.

Here's how:

1. Press the Windows key + R to open the RUN command.
2. Type %TEMP% and press Enter.
3. Right-click an empty area of the temp folder and select Properties.
4. Select Security.
5. Make sure all user names and groups in Security have Full Control.
6. Once the permissions have been set to Full Control, save it as a PDF again in QuickBooks.

Additionally, I'd like to add this article to learn how to fix unrecoverable errors for different scenarios: Unrecoverable error in QuickBooks Desktop.

Furthermore, you can also learn how to use and customize form templates for your transactions in QuickBooks Desktop.

Let me know if you have additional concerns besides needing help to email an invoice in QuickBooks Desktop. I'm always here to assist you.