A system and method are disclosed for registering a email sender for the purpose of sending an email message to an email receiver including receiving a challenge message wherein the challenge message includes a machine answerable question; processing the challenge message to determine that it is a challenge message sent for the purpose of authorizing delivery of the email message; analyzing the question to determine a valid response to the challenge message; and sending the valid response that includes the answer wherein a nontrivial amount of resources are required to send the valid response.