How do I know if buyers receive Order Confirmation email when testing with PayPal Sandbox

I use the following plugin "Configure SMTP" to send email from WordPress.

I set up SMTP in one sub blog dashboard, then sent at test email from there, and the test email was received by admin's email fine.

Now, I am testing Chained Payment with PayPal sandbox. You know, sandbox gives me a fake email, to check out in PayPal. The transaction was successful (I checked Sandbox account and MarketPress mange orders page).

My questions are:

1. Where does MarketPress send the email "Your Order Confirmation (ORDERID)" to? To user's email that he/she used to sign up new account in Wordpress? or his/her paypay account email?

2. If this email should be sent to user's wordpress account email, the email was not received for my WPMU site. Where should I look into for the problem? Is it because of the plugin "Configure SMTP" I use?

Thank you!