If you don’t have IPN turned on already, enable it with https://clients.yoursite.com/paypal/ipn as the notification url. The actual link you use here doesn’t matter as long as IPN is enabled.
In SPP there are a two settings you can configure for this module:
Seller email – the email address where you’ll be receiving payments (which clients will see).
Primary email – if your PayPal account has multiple emails enter your primary email here. SPP needs it to verify IPN notifications which only use the primary email.
If the system receives a payment notification from an email not in these settings, you will receive a security alert.