rpress_payment_user_email

This filter is applied to use the retrieve payment email, it is including two parameters.
$email -> is to get the email.
$ID  -> is used to get id.

function get_payment_user_email( $email ){
//its return email id of user
return $email
}
add_filter( 'rpress_payment_user_email', 'get_payment_user_email',10,2 );