Welcome to this thread, sreeder.

Let's try a few trobleshooting steps to resolve this issuw when sending invoice from QuickBooks Desktop (QBDT). Let's begin by making sure that your QuickBooks is updated to its latest release. Here's how to update your QuickBooks Desktop:

1. Go to the Help menu and select Update QuickBooks.
2. Choose the Options tab.
3. Click the Mark All button, then Save.
4. Click Update Now.
5. Check the Reset Update box, then click Get Updates.
   

Then, you can set up a secure WebMail to provide an easier and safer connection to your email provider.

Here's how:

1. Go to the Edit menu.
2. Select Preferences.
3. Tick Send forms.
4. Tap My Preferences and choose WebMail.
5. Hit the Add button.
6. Enter your email address in the Email Id field and select Gmail from the Email Provider drop-down.
7. Click OK.

To learn more about connecting your email to QuickBooks, here's an article made handy for you: Connect your email to QuickBooks Desktop.

Just in case you run into a connection error to your email server, these articles would be a great help:

•  Fix Error: Could not connect to the email server
• QuickBooks Will Not Accept My Web Mail Password

If you're still unable to do this step, I recommend contacting our Technical Team. You can reach them through chat or messaging. Make sure to enable pop-up windows on your browser settings to launch the chatbox and connect with our live agents.

To reach us, follow the steps below:

1. Open QuickBooks and click the Help menu.
2. Select QuickBooks Desktop Help.
3. On the pop-up screen select Contact us.
4. Click the Search for Something else button then type your concern in the Ask us anything box.
5. Hit Search then scroll down to get your contact options.

Also, to ensure we address your concern, our representatives are available from 6:00 AM to 6:00 PM on weekdays and 6:00 AM - 3:00 PM on Saturdays, PST. See our support hours and types for more details about this one.

Additionally, here's a link that will help you learn more about how the product works: Help articles for QuickBooks Desktop.

Be sure to get back to me if you have other concerns. I'd always here to assist you further. Wishing you and your business a continued success