address buyer checked code codes delivery email generated handful happened message minute notification product return send server successful times transaction unable

Why do we keep getting this notification below? It has happened a handful of times, though it seems a successful delivery message was sent to the buyer within the minute following the message.

"This is a notification that for the transaction (transaction ID), we were unable to send code(s) to the buyer (email address) as your server did not return anything."

Please make sure that you don't have "generated codes" checked for your product.