ok, So this is using PayPal Payments standard.
The limitation to paypal payments standard is that it is not a real time payment processor. When the user is returned to your site, the payment has been added to a queue to be processed on a first come first served basis, there is no way to know the status of the order.
Paypal will send the order receipt when the payment has been processed.
If you wish to set up a UE order receipt, you would need to configure an IPN page and add Universal EMail to the IPN Page to send the email when the order has been successfully processed.
For this reason, I recommend payments pro over payments standard.