Customers not receiving autoresponder email

I am having many customers complain that they are not receiving their autoresponder email. It's not in their spam folder. That email has their payment receipt on it! Big problem that needs a fast fix!!

Would you allow us to do a real submission in your form in order to test it?

Yes, but I'm not sure how you can do that without entering a credit card. 

I did temporarily disable the required validation on the payment field for this test. The autoresponder is working fine, I received the autoresponder without an issue:

How many people has reported you this problem? If you could let us know what their email addresses are, we could check if there is something in our mail logs.