Message challenge response

A system of registering an electronic-mail (e-mail) sender for the purpose of sending e-mail messages to an e-mail recipient, the system comprising:

• a network interface for receiving an e-mail sent by the e-mail sender to the e-mail recipient;
• an independent third-party auditor server for:
• executing instructions stored in memory, wherein execution of the instructions by a processor of an independent third-party auditor:
• generates a challenge message in response to an e-mail message received by the e-mail recipient, wherein:
• the challenge message includes a machine answerable question, a nontrivial amount of resources are required to determine a valid response to the machine answerable question, and
• the non-trivial amount of resources is adjustable by the independent third-party auditor;
• sending the challenge message to the e-mail sender;
• receiving a response to the challenge message sent by the e-mail sender, wherein the validity of the response is evaluated by the independent third-party auditor;
• and
• sending a report from the independent third-party auditor to the e-mail recipient, the report indicating the validity of the response, wherein the e-mail recipient registers the e-mail sender based on the report.

A non-transitory computer readable storage medium having embodied thereon a program, wherein the program is executable by a processor to perform a method of registering an electronic-mail (e-mail) sender for the purpose of sending e-mail messages to an e-mail recipient, the method comprising:

• receiving an e-mail sent by the e-mail sender to the e-mail recipient;
• generating a challenge message, using an independent third-party auditor, in response to an e-mail message received by the e-mail recipient, wherein:
• the challenge message includes a machine answerable question,
• a nontrivial amount of resources are required to determine a valid response to the machine answerable question, and
• the non-trivial amount of resources is adjustable by the independent third-party auditor;
• sending the challenge message to the e-mail sender;
• receiving a response to the challenge message sent by the e-mail sender, wherein the validity of the response is evaluated by the independent third-party auditor;
• and
• sending a report from the independent third-party auditor to the e-mail recipient, the report indicating the validity of the response, wherein the e-mail recipient registers the e-mail sender based on the report.