Some Emails Sent, others not

I am getting the "could not instantiate email" error message when trying to order. I have read all the posts about this and yes, Joomla's email function is operating correctly as I do get system emails and user reg emails.

I do get an email at the store "admin" address when the order is created or entered in the cart. But an receipt email does not sent back to the "user" (or me, in this case, tested with many different email addresses) with the order details. Also, status update emails do not get sent, and fail with the same message.

So, though I know emails can be sent properly, the end user is not getting their copy of the email, but the store admin is.

Thoughts?

Also have discovered that if I am logged into the frontend, email error message is not upon ordering, but still I do not get a receipt email either. However, the site admin account does, like noted above.

In any case, only emails to the site admin go though, but emails to the customer do not, showing the error.