I want to ensure you'll be able to send invoice in your QuickBooks Desktop (QBDT), @626.

To begin, let's ensure that your QBDT program is updated to the latest release. This is to make sure that you have the updated patches and fixes.

Then, let's now check your email settings to verify it does not block QuickBooks. This method varies depending on your email service. Please follow Step 2 of this article for the instructions: Fix webmail password issues in QuickBooks Desktop. 

Once done, check your antivirus software. This may accidentally block the connection. To do so, search for your antivirus provider's website for steps to allow port exceptions. For more information, visit this link: Fix webmail password issues in QuickBooks Desktop.

When everything is set, email the invoice again.

You can also customize your email templates to make them more personalized when sending forms to your customers. For the detailed steps, check out this article: Create custom email templates in QuickBooks Desktop.

Feel free to post again here if you have further concerns sending emails in QuickBooks. I'm just a post away to help. Take care always.