Why email auto-responder is not being sent when using PayPal in my form?

Could you confirm that that's the case?

Emails are delivered just fine with the Test Email button.

Emails are delivered just fine if I remove the paypal control from my form all together and submit.

Email to the owner goes out no problem in all cases.

I have a suspicion that emails are going out to the fake paypal sandbox email, instead of the one configured in the settings...

Left the form in the sandbox mode for today, if you'd like to try submitting.

I have checked your form and noticed that you have some incomplete payments, they seem to be some tests that you made on your form, but I'm not sure; however, i did a test and I did not make the payment when I was redirected to the PayPal payment page, I closed the tab and checked your submissions, completed manually the submission, as it was added to the incomplete payments, and I received the auto-responder email. 

Incomplete payments you have in your form.

Auto-responder email that I received when tested your form: 

Follow the steps on the guide below to view your incomplete payments: 

How-to-View-Incomplete-Payments

I also would like to suggest checking this link about incomplete payments on PayPal: 

Paypal-Incomplete-Payments-FAQ

Also, please enable the IPN(Instant Payment Notification) on your account so you will know each time you receive a payment: 

How-to-Enable-IPN-for-Paypal

It is good to set up an email notification each time you there is an incomplete payment as well, the guide below will help you with that: 

How-to-Setup-Incomplete-Payment-Notification-E-Mail

Now you will know either if you receive a payment or not, please try making some tests, an complete manually the incomplete payments, email notifications and auto-responders should be sent. 

Do get back to us if you need further assistance, we will be glad to help. 

Kevin,

Thank you for your quick response and for getting me on the right track.

The reason I was not getting all the notifications is because of this setting (which is exactly how I want it, I guess):

I checked, and yes, my IPN setting was set to OFF. I switched it to on, and tried again.

But alas, still got an uncompleted payment. Yes, once I clicked Complete payment manually, all emails showed up. 

Do you know if sandbox environment sends out ipn as well? Does it have a separate setting somewhere?

I am not really sure if IPN works with Paypal sandbox, you may ask your Paypal support.

Very well :)

Please let us know if there is more we can assist you with.

Cheers