Not sending "receipt" email to customer

I have been trying to fix this issue but I still can not figure out...
When I get orders, I receive the "receipt" emails from zencart but customers don't get any.
I tried smtp/send mail and both are fine.
I can send emails from the form on admin panel to the customers. Also "contact us" form works fine. I use authorize.net to accept credit card payment. I receive emails from them as it processed. However, for some reason, customers don't get any "receipt" emails, which are supposed to be sent as checking out successfully...
Does anyone know what causes this problem?

Please help me!

Did you turn it on in the email settings to send one to the customer?

Send E-Mails
Send out e-mails

Needs to be true ...

1. Do these "same" customers receive emails you send from Admin via Tools->Send Email?

2. What email addresses are they using? Are they free-mail accounts like yahoo/hotmail? If so, are your messages landing in their Junk-mail or Bulk-mail folders?

3. If you place a test order using your own email address (the same one where you're receiving your admin-receipt-copy emails), do "you" get your order-confirmation receipt as a customer? If so, then the problem is in how your customer is processing the email received.

4. What is your setting for Admin->Configuration->Email Options->Email Transport method?