Click on File, Business Info, Email tab.
The option for 'Use default email program when sending emails from client card" allows you to send emails through your computer's default email program.
This only applies to email sent through the customer card by clicking the Email button. It does not affect confirmation emails, reminders, birthday or expired vaccine emails, bulk emails, and so on. Those emails are sent using the email program you enter in the Email tab.
If you are not sure what your default email program is, it is the same program that will open if you click on an email hotlink on a website,