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I see that you have properly set up your Paypal integration properly with an email address set in it.
But if what you mean by receiver's email address is the where your purchasers put their email for making payments, users put it through Paypal's website after making a submission. Basically, your form will redirect to Paypal after they make a submission where they can log in and proceed with making payment. After completion, the user will be redirected back to your form and then your form processes the redirection to your thank you page setting.
However, if I am still misunderstanding your concern, please do let us know.